Potential Roles of Extracellular Vesicles as Diagnosis Biomarkers and Therapeutic Approaches for Cognitive Impairment in Alzheimer’s Disease

Abstract

Cognitive dysfunction, the major clinical manifestation of Alzheimer’s disease (AD), is caused by irreversible progressive neurological dysfunction. With the aging of the population, the incidence of AD is increasing year by year. However, there is neither a simple and accurate early diagnosis method, nor an effective method to alleviate or prevent the occurrence and progression of AD. Extracellular vesicles (EVs) are a number of heterogeneous membrane structures that arise from the endosome system or shed from the plasma membrane. In the brain, almost every kind of cell may have EVs, which are related to cell-cell communication and regulate cellular function. At present, an increasing body of evidence suggests that EVs play a crucial role in the pathogenesis of AD, and it is of great significance to use them as specific biomarkers and novel therapeutic targets for cognitive impairment in AD. This article reviews the potential role of EVs as diagnostic biomarkers and treatments for cognitive dysfunction in AD.

Keywords

Alzheimer’s disease cognitive impairment diagnosis biomarkers extracellular vesicles therapeutics

INTRODUCTION

Alzheimer’s disease (AD) is an irreversible neurodegenerative disease characterized by memory loss and cognitive disorder, which accounts for a large proportion of dementia cases [1 –3]. It is the commonest progressive neurodegenerative disease related to learning and memory defects caused by neurologic dysfunction [4]. Typical clinical manifestations of AD are progressive memory decline and executive dysfunction. Atypical clinical symptoms mostly appear in the non-memory field, which are recognition impairment, aphasia, and executive dysfunction [5]. AD is characterized by intracellular neurofilament tangles composed of abnormally phosphorylated tau protein aggregates and extracellular amyloid-β (Aβ) deposition, neuronal and synaptic loss, and neuroinflammation caused by reactive astrocytes and microglia [6, 7]. Aβ and tau, two of the hallmark pathologic proteins of AD, have long been related to neuron injury and death, contributing to gradual memory degeneration and severe cognitive impairment [8]. AD has been lurking decades before it is diagnosed as dementia the same as most chronic diseases. This long pre-clinical phase developed into a mild cognitive impairment (MCI) stage [9]. MCI is defined as the change of cognitive ability compared with the previous cognitive level. There are usually slight problems in completing complex functional tasks, which is a slight cognitive change rather than delirium [10]. Therefore, MCI is considered to be the earliest clinical manifestation of AD, with 80% of patients developing AD within 6 years [11]. Currently, the diagnosis of AD mainly includes cognitive tests, brain functional imaging, and evaluation of tau protein levels in cerebrospinal fluid (CSF) [10, 12]. However, the high cost of brain imaging and the invasiveness of CSF collection limit the possibility of its application in routine clinical screening and follow-up evaluation. At present, detecting the insoluble Aβ in extracellular plaque and tau proteins in intracellular tangles after death is the exact diagnosis of AD [13, 14]. In terms of treatment, the drugs currently approved by the FDA for patients with AD include partial N-methyl-D-aspartate receptor antagonists (memantine) and cholinesterase inhibitors such as rivastigmine, galantamine, and donepezil [15]. Unfortunately, those drugs have not succeeded in preventing the development of the disease though enhancing the quality of life and prolong life [16]. The late stage of AD often results in severe cognitive impairment, which places a heavy burden on the patients’ families and public administrators [17, 18]. On 7 June 2021, the US FDA announced the approval of Biogen (USA) Inc. and Eisai (Japan) Ltd. for Aduhelm (aducanumab-avwa), an antibody to Aβ, for the treatment of patients with early stage AD. However, it remains to be seen whether it will have the desired therapeutic effect. Therefore, the continued search for early diagnostic markers and the development of appropriate therapeutic agents remains crucial.

Extracellular vesicles (EVs) are small membrane bound vesicles with lower immune activity than their cell sources [19] and were first reported as cellular waste in the 1980s [20]. A number of studies have confirmed the existence of EVs in most biological fluids to date. EVs are now regarded as a supplementary intercellular communication pathway, functioning as an exchange of lipids, proteins, and various genetic material between cells [21]. Abnormal proteins folding are features of many neurodegenerative diseases such as AD. These pathogenic proteins (e.g., tau proteins and Aβ) are transmitted through the central nervous system (CNS), which is associated with exosomes in both normal and disease conformations, suggesting that EVs may be involved in the transmission of these pathogenic proteins [22, 23] and play a crucial part in the pathogenesis of these diseases [24]. More and more evidence supports that EVs can play a central role in the pathophysiology of AD as a carrier of pathogenic protein transfer between cells in the brain [25, 26]. Therefore, EVs could be used as a biomarker for the diagnosis and therapeutic monitoring of cognitive dysfunction in AD.

Based on the above background, the aim of this paper is to review the latest understanding of the multiple roles of EVs in AD-related cognitive dysfunction.

GENERAL CHARACTERISTICS OF EVs

Biological origin of EVs

EVs are membrane-restricted vesicles of a heterogeneous family originating from the endosome or plasma membrane [27]. The secretion of EVs (cytoplasmic vesicles in secretory cells encased in lipid bilayers) appears to be a conserved process during evolution [28]. Such vesicles can be released into the extracellular environment by different biological cells, including all eukaryotes and prokaryotes. Each cell regulates the biogenesis of EVs according to its physiological function, releasing EVs composed of specific lipids, proteins, and nucleic acids [29]. All cells can secrete various types of EVs, except for secreting vesicles released by specialized cells carrying hormones or neurotransmitters. This process has been preserved throughout evolution from bacteria to humans and plants [30 –32]. EVs are classified into two types: exosomes (30–150 nm) and micro vesicles (MVs) (200–500 nm) based on size and biogenesis [33, 34]. The biogenesis of exosomes and MVs is different, but neither can be separated from membrane transport. MVs and exosomes are secreted at different locations in the cell. MVs bud directly from the plasma membrane, whereas exosomes are represented by small vesicles of different sizes that bud into early endosomes and multivesicular bodies (MVBs) to form intraluminal vesicles (ILVs) and are released by fusion of MVBs with the plasma membrane [35 –38]. The exact steps of their occurrence are shown in Fig. 1.

Fig. 1

Schema of the synthesis of exosomes and micro vesicles. First, invagination of the plasma membrane produces early endosomes. The early endosomes then sprout inward a second time, producing intraluminal vesicles (ILVs), which become late endosomes or multivesicular bodies (MVBs). Then MVBs fuse with the lysosome and the contents of ILVs are degraded. Alternatively, MVBs can release ILVs into the extracellular space by fusing with the plasma membrane, where they are considered exosomes. MVs arise from the budding of the plasma membrane.

Characteristics of EVs

EVs consist of membranous vesicles of different origin, which are typically between 50 and 500 nm in size. Studies of the composition of EVs have shown that they can carry a variety of bioactive cargoes, including proteins, lipids, and nucleic acids [39], the amount of which varied considerably depending on different cells and environments. The differences in the substances carried explain their specific fate and function. Despite the differences in biogenesis, nature, and function [40], exosomes and MVs are similar in appearance, size, and composition. Once isolated from extracellular medium or biological liquid, it can be difficult to determine their origin. Th $\overset{´}{e}$ ry et al. used electron microscopy to observe isolated and purified exosomes. The results showed that the purified exosomes were 50–100 nm vesicles surrounded by a cup-like lipid layer with exosomal proteins on the surface [41]. Indeed, as there is no consensus on specific markers for EVs subtypes, such as endosomal-derived ‘exosomes’ and plasma membrane-derived ‘MVs’ [42, 43], assigning EVs to specific biogenic pathways remains difficult unless EVs are captured during are captured by real-time imaging techniques during release. Therefore, it is currently recommended to use the size (“small EVs” (< 100 nm or < 200 nm) and “medium/large EVs” (> 200 nm)), density (low, medium, high), biochemical composition (CD63/CD81 EVs, membrane-linked protein A5-stained EVs, etc.), and cells of origin (podocyte EVs, hypoxic EVs, large tumor vesicles, apoptotic vesicles) instead of terms such as exosomes and MVs to classify EVs [44].

Fig. 2

Schema of individual exosome and interactions between EVS and recipient cells. Recognition of EVs is achieved by the expression of tetrasomal proteins (e.g., CD9, CD81, and CD63), lipid rafts and flotilin 1 proteins. EVs of different cellular origin may have different cell-specific receptors. EVs contain DNA, RNA, and proteins that are specially packaged during assembly. EVs can be regarded as signaling bodies for a variety of biological processes. They are involved in antigen presentation and transport of major histocompatibility complex (MHC) molecules as well as antigens involved in immune regulation. EVs can directly activate cell surface receptors via proteins and bioactive lipid ligands, transfer cell surface receptors or deliver effectors such as transcription factors, oncogenes, and infection particles to recipient cells [82]. In addition, EVs contain a wide variety of RNAs, including mRNAs and small regulatory RNAs (e.g., miRNAs and non-coding RNAs), which are functionally uploaded to the recipient cell [83].

Physiological and pathological functions of EVs

As mentioned earlier, EVs are important communication mediators between cells and are carriers of proteins, lipids and nucleic acids that transmit biological signals between cells [45]. They are involved in normal physiological and pathological processes [46 –49] and play a double-edged role in the process of some neurodegenerative diseases. On the one hand, in neurodegenerative diseases, they act as the expansion products of pathogenic molecules [50], aggregate tau proteins [51] and initiate the replication of these proteins [52], and also act as carriers of tau [53, 54] and Aβ to facilitate their pathological dissemination. Exosomes secreted by disease cells may disrupt neurogenesis and thus participate in the progression of AD and nerve cell damage. For example, Eitan et al. found that AD-derived EVs promote intercellular transmission of Aβ, disrupting Ca^2 + handling and mitochondrial function of neurons and causing neuronal destruction [55]. On the other hand, since EVs are found throughout the brain, some specific sources of EVs may have disease therapeutic effects by acting on Aβ [56] to attenuate CNS damage [57].

In addition, there is growing evidence for the presence of intact mitochondria or mitochondrial components in EVs. Mitochondria in cells can stably generate mitochondria-derived vesicles (MDVs) [58], which are enriched in mitochondrial proteins. Most mitochondrial protein-rich MDVs (e.g., Tom20 and other oxidized cargo proteins) can be delivered to the endosome and fused to form MVBs [59, 60], which fuse with the plasma membrane to release cytosolic exosomes [61]. This phenomenon is a mitochondrial quality control strategy for dealing with damaged mitochondria [62] and also contributes to the intercellular transfer process of healthy mitochondria [63, 64], and autologous mitochondrial intercellular transfer is a therapeutic strategy based on the biologically expected recovery of pathological alterations in the recipient cells [65, 66]. mtDNA is present inside and/or on the surface of EVs. It has been shown that the amount of mtDNA in plasma EVs correlates with age and decreases with increasing age [67, 68].

EVs play an important role in the brain. Several studies have shown that EVs have the ability to cross the blood-brain barrier in both directions, although the pathway of transfer remains unclear [69, 70]. EVs can enter the brain parenchyma of the choroid plexus and mediate the import of folate into the brain [71]. Neuronal release of EVs is associated with the transfer of biomolecules across synapses and is thought to mediate synaptic plasticity in vertebrates and invertebrates. In rodents, glutamatergic synaptic activity triggers EVs release primarily from somatic dendritic (postsynaptic) sites [72, 73]. These EVs contain neurotransmitter receptor subunits, which leads to the implication that EVs release may affect the local elimination of these receptors from the postsynapse and, therefore, may modulate synaptic strength as part of a process known as steady-state synaptic scaling. Furthermore, neuronal EVs released in an activity-dependent manner carries synaptic plasticity-associated proteins and miRNAs [74] and preferentially interacts with target neurons at the presynaptic terminal [75]. EVs have been shown to promote cerebral angiogenesis, inhibit neuronal apoptosis, and reduce neuroinflammation. They promote neurite growth and axonal regeneration in damaged neurons [76] and play a broad role in neural activation, development, and regeneration.

Different types of cells within the brain can secrete EVs in the form of exosomes or microvesicles. Microglia are phagocytic cells that contribute to homeostasis within the CNS, and EVs are shed from their plasma membranes. Microglia-derived EVs regulate neurotransmission at excitatory glutamatergic and inhibitory gamma-aminobutyric acid (GABA)-ergic synapses primarily through lipid-mediated signaling [77, 78]. In addition, microglia-derived EVs are thought to propagate inflammation in the CNS because they carry the proinflammatory cytokine IL-1β and are increased in the cerebrospinal fluid [79]. EVs are involved in the intercommunication between myelinated oligodendrocytes and neurons. Electrically active neurons can trigger the release of oligodendrocyte EVs via neurotransmitter signals and internalize these EVs through endocytosis. Oligodendrocyte EVs not only provide nutritional support to target neurons and mediate neuroprotection, but also activate pro-survival signaling pathways and regulate gene expression in target neurons [80]. Astrocyte-derived EVs shed by astrocytes contain a large number of neuroprotective compounds, including fibroblast growth factor-2, vascular endothelial growth factor, and apolipoprotein D. Stimulated astrocytes secrete EVs with neuroprotective molecules, including heat shock proteins, synaptophysin 1, unique miRNAs, and glutamate transporters. Well-characterized astrocyte-derived EVs produced under specific culture conditions and astrocyte-derived EVs designed to carry the desired miRNAs or proteins may be used for the treatment of brain injury and neurogenic disorders [81].

EVs AS BIOMARKERS FOR COGNITIVE IMPAIRMENT IN AD

Biomarkers are necessary to improve the sensitivity and specificity of diagnosis and to monitor the biological characteristics of AD before clinical symptoms occur. It has been found that EVs can be released by CNS cells such as neurons, microglia, astrocytes, and oligodendrocytes [84], and can appear in the blood circulation by crossing the blood-brain barrier (BBB) [85]. They can be detected in serum, plasma, CSF, urine, saliva, breast milk, and other secretions [86]. The composition of EVs can reflect the physiological status of its parental cells [87]. Extracellular vesicle proteins and miRNAs as biomarkers for many neurological disorders [88]. therefore, EVs have a promising future as a new clinical diagnostic biomarker [89,90, 89,90] for cognitive impairment in AD.

The form of Aβ associated with AD pathophysiology can be isolated from human CSF EVs. EVs in CSF are a reliable biomarker with high specificity and sensitivity [91]. It can be used as a diagnostic marker to monitor cognitive impairment in AD [92]. However, as peripheral blood is more readily available than CSF, blood biomarkers are more advantageous in clinical diagnosis or screening. The ability of EVs to cross the BBB has great potential as a diagnostic biomarker for AD cognitive impairment [93]. Several studies have shown that blood EVs may be an ideal biomarker carrier for AD [94,95, 94,95]. Alterations in the morphology, number, and pathological protein levels of plasma exosomes may contribute to the diagnosis of AD. Patients with AD have smaller exosomes and fewer exosomes, which may provide a basis for the early diagnosis of AD [96].

Perrotte et al. found that major AD-related proteins were highly enriched in plasma EVs (pEVs) compared to plasma. They quantified AD-related proteins in pEVs isolated from normal controls, MCI, and AD subjects. In pEVs, they observed early changes in total tau protein (tTau), amyloid-β protein precursor levels and the phospho-tau protein (pTau)-T181/tTau ratio in MCI subjects. The results highlighted the modifications in some AD-related proteins in pEVs and suggested that levels of some of these proteins may be associated with cognitive decline. pEVs may be used as a biomarker to identify MCI and as a sensitive tool in dementia prevention tests [97]. A meta-analysis conducted by Liu et al. showed that decreased levels of neurogranulin in plasma exosomes of subjects progressing from MCI to AD (MCI-AD) were highly correlated with cognitive decline. These findings provide a clinical rationale for blood exosomal neurogranulin as a cognitive marker for MCI-AD [98]. Similarly, Kim et al. compared the RNAs content of pEVs in age-matched individuals with normal cognition and those with MCI due to AD. Using RNA sequencing analysis, they found that mitochondrial (mt)-RNAs was significantly elevated in pEVs from patients with AD cognitive impairment compared to the normal group. They are also proposed that in the AD brain, toxicity-induced mitochondrial damage results in the packaging of mitochondrial components for export into EVs, further suggesting that mt-RNA in pEVs could be used as a diagnostic and prognostic biomarker for cognitive impairment in AD [99].

Decreased plasma neuron-derived exosome levels of certain specific excitatory synaptic proteins (GluA4-containing glutamate and neuroligin 1) may indicate the degree of cognitive impairment and may reflect the severity of cognitive impairment in AD [100]. Synaptic protein plasma neuron-derived exosomes may be a useful preclinical marker and measure of AD progression [101]. Kapogiannis et al. validated biomarkers of plasma neuron-enriched EVs and further demonstrated that higher p-tau181 was associated with poorer verbal memory, attention, executive function, and visuospatial function. Higher Aβ₄₂ values were associated with better non-verbal memory and language. These findings have contributed to the further development of clinical blood tests for biomarkers of EVs in AD cognitive impairment [102]. In addition, it has been shown that the level of complement proteins in plasma astrocyte-derived exosomes are a component of neurotoxic inflammation and may be a predictive biomarker for the conversion of MCI to AD [103]. Patterson et al. identified receptor for advanced glycation endproducts in plasma neurogenic exosomes as a possible novel biomarker reflecting cognitive impairment in AD [104]. Hence, neuron-derived EVs have the potential to be a useful biomarker of cognitive impairment in AD [105,106, 105,106].

Studies have shown that circulating exosomal microRNAs (miRNAs) are not only potential biomarkers for early diagnosis of cognitive impairment in AD [107,108, 107,108] but may also provide new insights for screening and prevention of cognitive impairment in AD [109]. Rani et al. analyzed plasma pre-miRNA levels in 97 patients with AD. They found that pre-miRNA-342-3p and pre-miRNA-125b-5p were elevated and showed a strong correlation with patients’ cognitive function [110]. Liu et al. found that blood exosomal miRNA-193b can play a role in the course of AD, and exosome miR-193b was significantly lower in MCI patients than in controls when compared with the blood pre-miRNA expression profiles of MCI and AD patients [111]. Liu et al. used ABCA1 (an ATP-binding cassette subfamily A1 exporter) as a tag to capture specific exosomes and detected miRNA-135a in serum exosomes labeled with ABCA1 and found that the serum levels of abca1-labeled exosome miR-135a were significantly higher in the MCI patient group compared to the control group [112]. miR126-3p, miR142-3p, miR-146a-5p, and miR223-3p correlated with disease severity. Neural damage, specific miRNA downregulation, and inflammatory cytokine upregulation, found in patients’ EVs, might be used as a biomarker reflecting AD severity [113]. miR-212 and miR-132 are downregulated in neurally derived plasma exosomes of AD patients [114]. The experimental results of G $\overset{´}{a}$ mez-Valero et al. [115] showed that a set of four miRNAs (hsa-miR-23a-3p, hsa-miR-126-3p, hsa-let-7i-5p, and hsa-miR-151a-3p) significantly decreased in AD respect to controls. Vogrinc et al. [116] prioritized EV-related miRNA biomarkers interacting with known AD-related genes and identified the most promising circulating miRNAs and their miRNA-target gene interactions that could serve as biomarkers for AD. miRNAs hsa-miR-193b, hsa-miR-29c, hsa-miR-613, hsa-miR-206, hsa-miR-128, and hsa-miR-146a represent the most promising circulating AD biomarkers, while miRNAs hsa-miR-375 and hsa-miR-107 could be promising EV-related AD biomarkers.

THERAPEUTIC EFFECT OF EVs IN COGNITIVE IMPAIRMENT OF AD

Although the pathogenesis of AD is diverse, neuron injury and synaptic impairment are the critical factors contributing to cognitive dysfunction. Thus, neuroprotection and neurogenesis will provide an important alternative for the recovery of AD cognitive function. Previous studies have shown that exosomes have the ability to reduce the deposition of Aβ in various ways. For instance, neuronal-derived EVs promoted the transformation of Aβ by driving the conformational changes to form non-toxic amyloid fibrils [117 –119]. Exosomes can rescue long-term enhancement from Aβ-mediated damage in vivo by isolating synaptotoxic Aβ oligomers through surface proteins such as prion protein. Exogenous exosomes act as powerful scavengers of Aβ in the mouse brain, offering a novel strategy for the treatment of cognitive impairment in AD [120]. In addition, the potential of EVs for drug transport has attracted great attention in this field, because these vesicles can bypass the main barriers related to drug delivery through BBB [121]. Thus, they can play an effective role in the treatment of AD cognitive impairment.

Mesenchymal stem cell-derived EVs

Now there is sufficient evidence that EVs secreted by stem cells can effectively reproduce its therapeutic effect, making stem cells-derived EVs an attractive off the shelf, cell-free therapy, which may be effective, safer, and cheaper [122 –124]. One study demonstrated that reverse osmosis treatment with human neural stem cell-derived EVs significantly reduced the accumulation of dense core amyloid plaques and microglia activation, eliminating fear, consolidating memory, and reducing anxious behavior. Importantly, EVs treatment prevents synaptic loss in AD brains, which is consistent with improved cognitive performance. Thus, systemic injection of stem cell-derived EVs is neuroprotective against AD neuropathy [125].

Mesenchymal stem cells (MSCs)-derived exosomes are also emerging as an attractive tool for the treatment of AD [126]. According to previous studies, they have the advantages of wide donor sources, low immunogenicity, easy storage, natural carrier, and low risk of tumor formation. EVs from bone marrow MSCs are effective in reducing Aβ plaque deposition and the number of dystrophic nerves, stimulating neurogenesis in subventricular regions [127], facilitating cognitive recovery, and playing a potential role in the early cognitive impairment in AD [128, 129]. Cui et al. found that the use of hypoxia-pretreated MSCs exosomes improved not only learning ability but also memory in AD mice [130].

Ma’s study confirmed the potential of adipose-derived MSCs-derived EVs (ADSCS-EVs) in AD treatment and revealed key mechanisms for their neuroprotection and promotion of neurogenesis. Following transnasal administration, EVs showed rapid and efficient effects. Proteomic analysis showed that EVs contain neuroprotective and neurogenic proteins. EVs treatment resulted in upregulation of neurogenesis or neurite growth-related and pro-survival gene expression and downregulation of pro-apoptotic gene expression. In addition, EVs were protective against Aβ_1–42 oligomers or glutamate-induced neurotoxicity. Intranasal administration of EVs effectively rescued memory loss, significantly improved neurological function, increased the number of newborn neurons, mildly reduced Aβ deposition, and decreased microglia activation levels in AD model mice [131]. These findings suggest that adipose-derived MSC-EVs have disease-modifying effects on AD and may act as a source of cure to ameliorate Aβ-induced neuronal death and the progression of cognitive impairment in AD [132, 133].

Losurdo et al. showed that intranasal administration of EVs from MSCs pretreated with cytokines could induce immunoregulation and neuroprotection in AD [134], and Wang’s data suggest that MSC-EVs may play a beneficial role in reducing synaptic plasticity and cognitive behavioral deficits in mouse models of AD [135]. MSC-EVs, as a novel cell-free therapeutic approach, have incomparable advantages compared with cell-based therapy, and is considered to be a promising choice for the treatment of AD [136]. Based on the advantages of MSCs as therapeutic vectors and the therapeutic effects of EVs derived from these cells, researchers have concluded that MSCs are an ideal source of EVs. As a result, MSC-EVs have become an attractive treatment for cognitive impairment in AD.

EVs as drug delivery systems

In recent years, there has been a growing interest in the role of natural compounds in the treatment of AD cognitive disorders. They have the advantages of good therapeutic effects and few side effects; however, the disadvantages of low solubility, low bioavailability, and not easily crossing the BBB exist as well. Exosomes are structurally simple, can be integrated into the plasma membrane, cross BBB and therefore have the opportunity to be used as carriers of drugs and genetic factors for the treatment of immune, psychiatric, and neurological disorders. Flavonoids are unique natural molecules with antioxidant and anti-inflammatory effects. Therefore, the design of flavonoid-containing exosomes is of great value [137]. Quercetin, a flavonoid natural compound, has been considered as a promising cognitive enhancer due to its potential neuroprotective, antioxidant, and anti-inflammatory pharmacological effects [138]. In particular, quercetin has been reported to prevent tau pathology, inhibit amyloid production, and induce neuroprotection [139]. However, its poor solubility, low bioavailability, and difficulty in crossing the BBB have hindered the clinical development of quercetin as a potential therapeutic agent [140]. Qi et al. developed plasma exosomes-loaded quercetin to improve the bioavailability of the drug, enhance the brain targeting of quercetin, and effectively improve AD mice with cognitive dysfunction [141].

Fig. 3

Schema of roles of EVs for cognitive impairment in Alzheimer’s disease (AD). EVs in blood, CSF, urine, saliva, and other body fluids can serve as biomarkers of cognitive dysfunction in AD. EVs derived from mesenchymal stem cells (MSC), microglia, neural stem cells (NSC), amniotic fluid stem cells (ASC), etc. can be used as a new treatment for cognitive dysfunction in AD.

In addition, curcumin, 1,7-bis-(4-hydroxy-3-methoxyph,enyl)-1, 6-heptadiene-3,5-dione, is a diarylheptanoid, belonging to the group of curcuminoids. It has a regulatory effect on tau phosphorylation [142], and previously reports indicated that curcumin can play a therapeutic role in AD by regulating tau phosphorylation [143,144, 143,144], similarly, curcumin still suffers from poor solubility and low bioavailability. Wang et al. prepared exosomes secreted by curcumin-treated mouse macrophages. The results showed that this approach improved the solubility and bioavailability of curcumin and increased the penetration of the drug in the BBB. Exosomes from curcumin-treated cells better prevented neuronal protein death in vivo and in vitro and improved learning memory deficits in AD mice [145].

Neprilysin, also known as cluster differentiation 10, is a major degradative enzyme with soluble monomers or oligomers of Aβ_1–42 (the most toxic form) and insoluble protofibrillar forms of Aβ_1–40 in the brain [146]. AD development is promoted by promoting plaque accumulation [147]. Thus, controlling the degradable (monomeric and non-toxic) form of Aβ peptides through neprilysin could be a preventive strategy for the clinical development of dementia [148,149, 148,149]. Unfortunately, proteins and a large number of effective drugs have been shown not to cross the BBB [150]. To overcome this problem, Izadpanah et al. used EV-loaded neprilysin to improve memory in AD. EVs-loaded neprilysin improved brain-related behavior functions, which may be mediated through modulation of inflammation and apoptosis. These findings suggest that EV-loaded neprilysin could be considered as a potential drug delivery system to improve AD.

DISCUSSION AND OUTLOOK

The first clinical manifestation of AD is MCI. If early cognitive impairment is not detected and managed in a timely manner, the progression of the disease can place a great burden on the patient’s family and society as a whole. Current diagnostic and therapeutic measures for AD are limited, diagnostic tools are complex and invasive, and routine screening is difficult to achieve, making early detection difficult and accurate.

The discovery of EVs, a heterogeneous group of cell-derived membrane structures that are secreted by various cells in the body and are often involved in disease transmission, has provided new ideas for the development of treatments for AD cognitive disorders [151]. In the pathogenesis of AD, EVs of neuronal origin play a role in promoting disease progression. However, EVs still have the following advantages: firstly, as they can penetrate the BBB, this allows one to detect neurogenic EVs in the blood circulation, which helps in the diagnosis of AD cognitive disorders, while EVs can deliver some insoluble and poorly bioavailable drugs in a minimally invasive manner (e.g., blood and/or intranasal delivery), which helps in the treatment of AD cognitive disorders. Secondly, we can adapt the composition, form, and cargo of EVs to specific needs for additional purposes. In addition, membrane proteins expressed by EVs can be designed to target precise cell types, improving specific treatment of AD cognitive impairment and reducing the incidence of side effects. In short, EVs have the following functions: maintaining cellular communication and function, acting as a diagnostic marker for certain diseases, transporting drugs, and facilitating disease treatment [152].

Current means of confirming AD have many drawbacks: invasive manipulation of CSF, potential biomarkers are susceptible to degradation by the underlying proteins and are usually present at low concentrations. EVs can be detected in the surrounding biological fluids due to their ability to cross the BBB, thus protecting their cargo from degradation [153]. EVs can overcome these challenges and therefore provide a very practical source of alternative biomarkers [154 –156]. As mentioned above, detection of serum ABCA1-labelled exosomes may help in the early diagnosis of AD cognitive impairment [157]. Serum miRNA is not only a potential biomarker for the early diagnosis of AD cognitive disorders but may also provide new ideas for the screening and prevention of AD cognitive disorders.

Obviously, the biomarkers of cognitive impairment in AD are currently more often studied in the context of neurogenic EVs in the circulation, and still to a lesser extent in the context of urinary exosomes [158,159, 158,159] and salivary exosomes [160]. In the study of AD cognitive disorders, it is believed that in the near future, not only blood EVs but even urinary EVs, salivary EVs, and fecal EVs can be used as biomarkers of AD cognitive disorders, providing more simple, effective, and fast ways to diagnose AD cognitive disorders.

In terms of treatment, in addition to the neuron-derived EVs and stem cell-derived EVs mentioned above, microglia-derived EVs, amniotic fluid stem cell EVs [161], and human umbilical cord MSC EVs [162] have also been found to be effective in alleviating neurodegenerative changes and cognitive dysfunction in AD patients [163]. The increasing number of studies using EVs as a drug carrier for precise delivery of anti-AD cognitive disorders drugs to the brain will open up more possibilities for the treatment of AD cognitive disorders in the future.

CONCLUSION

EVs play a pathogenic role in the development of AD cognitive disorders as disease-transmitting intercellular communication substances and can also be used as diagnostic markers for AD cognitive disorders. Both in their own therapeutic role and as drug transporters, exogenous EVs offer additional possibilities for the treatment of AD cognitive disorders.

Footnotes

ACKNOWLEDGMENTS

We sincerely thank all the staff who participated in this review.

This work was supported by the National Natural Science Foundation of China (grant no. 81773988 and 82073923) and the Provincial Natural Science Research Project of Anhui Colleges, China (grant no. KJ2020A0431).

Authors’ disclosures available online ().

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