The role of exosomes in adult neurogenesis: implications for neurodegenerative diseases

Abstract

Exosomes are cup-shaped extracellular vesicles with a lipid bilayer that is approximately 30 to 200 nm in thickness. Exosomes are widely distributed in a range of body fluids, including urine, blood, milk, and saliva. Exosomes exert biological function by transporting factors between different cells and by regulating biological pathways in recipient cells. As an important form of intercellular communication, exosomes are increasingly being investigated due to their ability to transfer bioactive molecules such as lipids, proteins, mRNAs, and microRNAs between cells, and because they can regulate physiological and pathological processes in the central nervous system. Adult neurogenesis is a multistage process by which new neurons are generated and migrate to be integrated into existing neuronal circuits. In the adult brain, neurogenesis is mainly localized in two specialized niches: the subventricular zone adjacent to the lateral ventricles and the subgranular zone of the dentate gyrus. An increasing body of evidence indicates that adult neurogenesis is tightly controlled by environmental conditions with the niches. In recent studies, exosomes released from different sources of cells were shown to play an active role in regulating neurogenesis both in vitro and in vivo, thereby participating in the progression of neurodegenerative disorders in patients and in various disease models. Here, we provide a state-of-the-art synopsis of existing research that aimed to identify the diverse components of exosome cargoes and elucidate the therapeutic potential of exosomal contents in the regulation of neurogenesis in several neurodegenerative diseases. We emphasize that exosomal cargoes could serve as a potential biomarker to monitor functional neurogenesis in adults. In addition, exosomes can also be considered as a novel therapeutic approach to treat various neurodegenerative disorders by improving endogenous neurogenesis to mitigate neuronal loss in the central nervous system.

Introduction

Exosomes are tiny, single-membrane, organelles that are approximately 30 to 200 nm in size and are secreted as extracellular cup-shaped discoid vesicles with a lipid bilayer (Pegtel and Gould, 2019). Exosomes were first discovered in reticulocytes by Pan and Johnstone in 1983 (Pan and Johnstone, 1983). In terms of topological structure, exosomes are similar to cells and contain a variety of selected proteins, lipids, nucleic acids, and glycoconjugates; all of these can be delivered to cells (Salimi et al., 2020). Many studies have revealed that exosomes represent a novel mode of intercellular communication and participate in a wide range of physiological and pathological processes (Zhang et al., 2014, 2016b; Cano et al., 2023). A growing body of evidence suggests that exosomes also play critical roles in neurodegenerative diseases (D’Anca et al., 2019; Izco et al., 2022; Jiang et al., 2022). Exosomes and their cargoes may serve as biomarkers for disease progression. Exosomes could also be used as therapeutic targets to modulate neuroinflammation in the central nervous system (CNS). Recent studies have shown that exosomes also participate in crosstalk between cells in the neurogenic niche, thus modulating the process of neurogenesis in the adult brain (Bátiz et al., 2015; Egeland et al., 2015; Li and Guo, 2021). In this review, we summarized the bioactive cargoes of exosomes that have been identified to modulate the proliferation and fate of neural stem cells (NSCs) in the brain, and describe the underlying mechanisms of neurodegeneration and neuroinflammation that can be regulated by the cargo contained in exosomes. We also highlight the therapeutic potential of exosomes for neurodegenerative disorders via the improvement of neurogenesis in adults.

Literature Retrieval Strategy

We used the Web of Science and PubMed databases to collate relevant full-text articles published from inception to January 31, 2023 and written in English. The literature search was conducted by combining a range of key words, including “adult neurogenesis”, “exosome”, “extracellular vesicles”, “neurodegenerative disorders”, “Parkinson’s disease”, “Alzheimer’s disease”, “Huntington’s disease” and/or,” amyotrophic lateral sclerosis” to limit the topics and maximize the specificity and sensitivity of the references identified. After acquiring the selected reference list, we screened the titles and abstracts to identify potentially useful studies. We also used the Web of Science database to identify the citation status of each reference and by determining how many times each reference had been cited, we were able to identify high-impact papers. Next, we used the PubMed database to access the full texts of useful studies. In particular, we included two aspects of studies in each category of neurodegenerative diseases; we included studies that focused on the relationship between exosome cargoes and neurogenesis, and studies that were related to exosomes derived from neural stem cells. For example, when considering the “Regulation of neurogenesis by exosomes in PD”, we collated studies that investigated the relationship between neurogenesis and exosomes in PD. However, we also included studies that focused on exosomes derived from other cell types and used to modulate neurogenesis for the treatment of animal model of PD.

Exosome Biogenesis

Exosomes were initially thought to be extracellular debris secreted by the reticulocyte substances that were destroyed by lysosomes (Kruh-Garcia et al., 2015). However, more recent studies have demonstrated that exosomes originate from the endocytic pathway (Lässer, 2015; Hessvik and Llorente, 2018) and that the typical process of exosomal biogenesis includes the formation of intraluminal vesicles and multi-vesicular bodies before extracellular release (Alenquer and Amorim, 2015; Yue et al., 2020). There are three main mechanisms involved in the formation of exosomes (DM and SJ, 2019): (1) vesicles bud into discrete endosomes that mature into multivesicular bodies and release exosomes as the plasma membranes undergo fusion; (2) immediate release from the plasma membrane by direct vesicle budding, and (3) delayed release by budding in the intracellular plasma membrane-connected compartment (IPMC) followed by contraction of the IPMC neck.

During the process of biogenesis, many types of proteins are loaded into exosomes, including membrane transporters and heat shock proteins (Jones et al., 2018). In addition, exosomes also contain a multitude of non-coding RNAs, including long non-coding RNAs (lncRNAs), microRNAs (miRNAs) and circular RNAs (circRNAs) (Li et al., 2021). Exosomes are enriched in body fluids, including urine, blood, milk, and saliva (Qin and Xu, 2014; Sun et al., 2018), and are frequently transported in the body via the blood circulation to exert their biological effects. As with the pathways by which viral particles are transported in target cells, exosomes are taken up by multiple processes, including macropinocytosis, phagocytosis, clathrin-dependent endocytosis, and clathrin-independent endocytosis (van Dongen et al., 2016). Target cells usually recognize and capture exosomes in three ways: (1) exosomes or their released substances bind to ligands on the surface of cell membranes; (2) exosomes are endocytosed by recipient cells, and (3) exosomes fuse with the membranes of target cells. Therefore, as shown in Figure 1, exosomes exert biological effects mainly by transporting bioactive materials such as proteins and miRNAs between cells.

Figure 1.

Exosomal cargoes.

Exosomes bud from the endosomes and plasma membranes of cells and are mostly made up of genomic cargoes and non-genomic cargoes. There are two main protein components: one of these is essential for exosomal production and the other is unique to cell derivation. The majority of proteins involved in the synthesis of poly-vesicular endosomes are ALG-2-interacting protein X (Alix), tumor susceptibility gene 101 (TSG101), chaperone heat shock proteins (HSP70, HSP90, and HSP60), the tetraspanin superfamily (CD9, CD63, CD81, and CD82), RAS-related protein (Rab), and lipid raft marker protein (Yoshioka et al., 2013). Cholesterol, sphingolipids, and ceramides are all examples of lipid components. In addition, DNA, mRNAs, microRNAs (miRNAs), circular RNAs (circRNAs), and long non-coding RNAs are abundant in exosomes. Created using Adobe Illustrator.

The Relationship between Exosomes and Adult Neurogenesis

It is widely acknowledged that NSCs are present in certain regions of the adult mammalian brain where they proliferate to produce new neurons, a process known as adult neurogenesis (Matsubara et al., 2021). It is estimated that 9000 new cells are generated every 25 hours in the rat hippocampus (Cameron and McKay, 2001). In humans, direct evidence of adult neurogenesis was first obtained by the analysis of post-mortem brain tissue from cancer patients treated with the thymidine analog bromodeoxyuridine (BrdU, 5-bromo-2′-deoxyuridine) (Eriksson et al., 1998). The regulation of adult neurogenesis is a complex process that involves extracellular factors and intracellular mechanisms in the brain; this is a complex topic and has become a significant hotspot in research. Most regions of the mammalian brain acquire intact neurons before birth, and each neuronal population is added during a specific developmental stage (Cameron and Glover, 2015). In the mammalian brain, new neurons arise predominantly in the subventricular zone (SVZ) of the ventricular lateral zone (Bonaguidi et al., 2011) and the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG) (Seri et al., 2001). Two significant properties of NSCs are cell proliferation and the production of three differentiated neural lineages: neurons, astrocytes, and oligodendrocytes (Gonçalves et al., 2016). NSCs can divide symmetrically or asymmetrically although the latter process is more dominant. Research has shown that the symmetrical division of a single NSC produces offspring and an immature neuron (Bonaguidi et al., 2011). These immature neurons migrate to the DG or olfactory bulb (OB) to further differentiate and mature into corresponding neural cells (Cope and Gould, 2019).

Many studies have demonstrated that the regulation of neurogenesis is dependent on the microenvironment (the neurogenic niche) of NSCs in the brain (Lepousez et al., 2013; Toda et al., 2019; Li and Guo, 2021). A distinct and specialized microenvironment known as the neurogenic niche encourages the proliferation and differentiation of NSCs toward the neuronal lineage and glia cells (Shin et al., 2014). As indicated in Figure 2, the neurogenic niche is known to be made up of a variety of cell types, including astrocytes, neurons, axon projections, and structures such as blood vessels. One of the key roles of the niche is to create an environment that will maintain most stem cells in a dormant and undifferentiated state. Signals from different cellular components within the niche also play a role in regulating the self-renewal and multipotent properties of NSCs (Liu et al., 2017).

Figure 2.

Neural stem cells in the neurogenic niche.

Neural stem cells are mainly found in the subventricular zone (SVZ) region of the lateral ventricle and the subgranular zone (SGZ) region of the dentate gyrus of the hippocampus in the adult mammalian brain under physiological conditions. In the process of neurogenesis, neuroblasts in the SGZ and SVZ migrate to the dentate gyrus (DG) and olfactory bulb (OB) regions in a short or long migrational manner, respectively, following early differentiation to further differentiate and mature into corresponding neural cells into related circuits. Neural stem cells, neural progenitor cells, neuroblasts, immature neurons, and mature neurons are successively produced. Created using Adobe Illustrator. RMS: Rostral migratory stream.

The Transmission of Exosomes in Neurogenic Niches

Intercellular communication in the neurogenic niche is crucial for dynamic balance and the regulation of adult neurogenesis, especially with regards to signaling molecules such as cytokines, neurotransmitters, and hormones. Since exosomes serve as a tool for epigenetic regulation and the delivery of bioactive proteins for intercellular communication (Li et al., 2022a), they may also provide a microenvironment that facilitates disease progression in the brain. For example, it has been shown that in the animal model of Alzheimer’s disease (AD), exosomes loaded with tau and α-synuclein (α-syn) were transported between neurons that were connected by synapses. The intracerebral injection of pathological tau from AD brains induces the seeding of normal tau in mouse brain (de Fisenne et al., 2022). In addition, researchers analyzed samples from the niches of adult amyotrophic lateral sclerosis (ALS), Huntington’s disease (HD), Parkinson’s disease (PD), Lewy body dementia, and frontotemporal dementia (Terreros-Roncal et al., 2021) and identified alterations in the ratio of quiescent to proliferating hippocampal NSCs along with alterations in homeostasis within the neurogenic niche. Studies have also demonstrated that aging and neurodegenerative processes can reduce the phagocytic capacity of microglia (Yanguas-Casás et al., 2020), trigger astrogliosis, and alter the microvasculature of the dentate gyrus. It is known that signal transmission between diverse glial cells and neurons in neurogenic niches is particularly critical for maintaining the dynamic balance of adult neurogenesis and the active regulation of neural plasticity (Terreros-Roncal et al., 2021). Many cytokines, neurotransmitters and hormones play a significant role in this process. Different cell types, including NSCs, neurons and glia, have all been shown to release exosomes in the niche and, in turn, exert impact upon complex niche environments (Pardal and Lopez Barneo, 2016; Men et al., 2019; Rong et al., 2019). An elegant review previously summarized our current knowledge of the physiological role of exosomes within the neurogenic niche and how they can modulate neurogenesis (Losurdo and Grilli, 2020). An increasing body of evidence also indicates that exosomes and their contents are also involved in the onset and progression of neurodegenerative diseases by modulating the microenvironment of the niche.

Conversely, recent studies have demonstrated that the non-niche cells derived exosomes also participate in the regulation of neuroinflammation; this can benefit the environment supporting the neurogenic niche, enhance neurogenesis, and may even have potential therapeutic applications for several neurological diseases (Luarte et al., 2016; Yang et al., 2017; Nasirishargh et al., 2021). For example, exosomes secreted by somatic cell-induced NPCs and normal NPCs have been shown to regulate neuronal differentiation and promote neural regeneration via miR-21a (Ma et al., 2019). Another study found that multiple miRNAs (miR-125, miR-145, miR-18, and miR-21) are closely related to the process of adult neurogenesis in human mesenchymal stem cell-derived exosomes (Lojewski et al., 2014). These authors investigated the potential effects of these exosomes on the differentiation of multipotent NSCs and found that the mRNA levels of the NPC marker nestin were elevated in NSCs exposed to exosomes from all sources of human mesenchymal stem cells. Based on these properties, these exosomes also have significant therapeutic potential for various neurological diseases.

The Role of Exosomes in the Regulation of Neurogenesis in Neurodegenerative Disorders

The regulation of neurogenesis by exosomes in PD

PD, the second most common neurodegenerative condition after Alzheimer’s disease, is clinically characterized by tremor, rigidity, bradykinesia and postural instability. The pathological characteristics of PD include the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc), severe misfolding and the aberrant buildup of α-syn in the remaining dopaminergic neurons, and the development of Lewy bodies (Mor et al., 2016). The tegmental aqueduct periventricular region (Aq-PVRs), which lies adjacent to the SNpc and contains clonogenic NSCs with DAergic potential, is thought to contain dormant neural progenitors (Marchetti et al., 2020). Adult Aq-PVR NSCs can be stimulated and induced to transform into DAergic neurons after PD injury both in vitro and in vivo (Marchetti et al., 2020); this represents an important sign for adult hippocampal neurogenesis in PD. A-synuclein, a well-known protein that controls adult neurogenesis, is essential for the development of PD and Lewy body dementia (Hall et al., 2014). When discharged into the extracellular area, α-synuclein can be taken up by neurons, NPCs, and astrocytes (Lee et al., 2010). Studies in transgenic rat models of PD have revealed that the accumulation of α-synuclein and the impairment of 5-HT neurotransmission have a deleterious impact on hippocampal neurogenesis in PD. This may occur before the development of protein aggregation and motor impairments in this disease (Kohl et al., 2016). After the typical period of dopaminergic neurogenesis, Lmx1a and other progenitor markers are still present in the midbrain aqueduct region, and dopamine receptor antagonists have been shown to promote their proliferation and improve neurogenesis (Hedlund et al., 2016).

Studies have also indicated that abnormal α-syn accumulation, oxidative stress, calcium homeostasis, and impaired axonal transport are the main causes of impaired neurogenesis in PD (Kline et al., 2021). Since exosome transmission between glial cells and neurons is actively involved in these processes, it follows that this mechanism is crucial for controlling neurogenesis in PD. There is evidence that α-syn aggregates can induce microglia to secrete exosomes containing α-syn oligomers. Exosome-associated a-syn oligomers are more likely to be received by nearby cells and trigger greater levels of toxicity when compared to the free oligomers (Danzer et al., 2012). Further studies demonstrated that pro-inflammatory cytokines can also trigger microglia to release more exosome-associated α-syn and enhance the inflammatory response in the brain (Guo et al., 2021). In such cases, exosomes could play a detrimental role in the transcellular spread of α-syn oligomers, thus impairing endogenous neurogenesis via toxic exosomal α-syn oligomers. More in-depth and detailed studies should aim to identify the function of exosome-associated α-syn in the regulation of adult neurogenesis in PD. Researchers have also tried to utilize exosomes generated from various stem cells in vitro to inhibit the aggregation of α-syn and stop the degeneration process in dopaminergic neurons. For example, some studies have indicated that exosomes also exhibit significant therapeutic potential to improve the function of neurogenesis in the niche. Exosomes derived from bone marrow mesenchymal stem cells have been shown to reverse the pathogenic features of PD by remodeling the inflammatory microenvironment in the SNpc region and by repairing DA nerve injury (Li et al., 2022b). Studies have also indicated that astrocyte atrophy in the early stages of PD may be pathologically relevant to the disturbance of exosome biogenesis in the niche, although their significance in disease progression remains unknown (Gómez-Gonzalo et al., 2017). Exosomes and microvesicles were previously generated from dental pulp stem cells and investigated for their ability to protect human dopaminergic neurons from oxidative stress under 6-hydroxydopamine treatment (Jarmalavičiūtė et al., 2015). Exosomes generated from dental pulp stem cells are thought to represent new therapeutic tools for the treatment of PD since the treatment of human DA neurons with these exosomes was shown to significantly reduce apoptosis induced by 6-hydroxydopamine.

The regulatory role of exosomes in the neurogenic niche in AD

AD is a neurodegenerative disease characterized by memory loss, cognitive impairment, and eventually, the loss of ability to carry out the simplest tasks. In 2013, a report on the epidemiology of AD was published by the World Health Organization; this report predicted that the global number of subjects with dementia could double by 2050 (Khan et al., 2020). Along with granulovacuolar degeneration and cerebrovascular alterations, there are two other key pathogenic abnormalities in AD: neuronal fibrillary tangles in brain cells created by tau hyperphosphorylation and senile plaques caused by the deposition of amyloid beta (Blennow et al., 2006). Neurodegeneration, with neuronal shrinkage triggered by the loss of synapses and cell bodies, is one of the progressive signs of AD (Schneider et al., 2009), especially in the hippocampus of AD patients (Benitez et al., 2015). Only a small number of studies have reported neurogenesis in AD even though research has shown that AD exacerbates the process of neurogenesis to a greater extent than physiological aging. A previous study used Mushashi as a marker and identified fewer progenitor cells in the SVZ of AD patients, despite an increase in nestin-positive stem cells (Babcock et al., 2021). Based on the expression of doublecortin (DCX), polysialic acid-neural cell adhesion molecule, and TUC4, a neurogenic differentiation factor, another study found that AD patients experienced enhanced neurogenesis in the hippocampus (Jin et al., 2004). Numerous studies employing transgenic mouse models of familial Alzheimer’s disease have identified changes and dysfunctions in the SGZ of the DG (Wilke et al., 2014). Early findings also revealed changes in the neurogenic niche of AD patients, thus indicating impaired neurogenesis. The proliferation and survival of NPCs, as well as the quantity of new neurons in the DG of the hippocampus, were reported to be affected at 3, 6, and 9 months of age when compared to non-transgenic littermates when using a Tg2576 transgenic mouse model overexpressing APP (Pan et al., 2016).

Reactive astrogliosis is a process that protects the CNS by isolating damaged areas and reconstructing the blood brain barrier to support neural circuit remodeling; this is also a typical morphological feature of the brain in the late stages of AD (Vardjan et al., 2015). A previous study also found that amyloid-beta (Aβ) can be internalized into astrocytes and may alter the dynamics of vesicles, thus promoting the development of AD (Nagele et al., 2004). The neurotoxicity of Aβ and cell-to-cell dissemination have been associated with exosomes. A recent study reported the exosome-mediated transmission of neurotoxic proteins in AD; this study found that exosomes can be loaded with oligomer Aβ, thus leading to the transmission of exosomal Aβ from astrocytes to nearby neurons (Sardar Sinha et al., 2018). By investigating the proteomic profiles of exosomes isolated from different human-induced pluripotent stem cell-derived neural cell types, researchers have also found that cell-type-specific exosomes are involved in the progression of AD. A protein module enriched in astrocyte-specific exosomal markers was shown to be significantly associated with the pathology of AD and cognitive impairment. Further research should aim to investigate the role of astrocyte-specific exosome cargoes in the neurogenic niche since astrocytes contribute to neurogenesis in two distinct pathways: as NSCs and as niche cells. The microglia serve as primary phagocytes in the brain and are capable of ingesting neuronal exosomes that carry intact and hyperphosphorylated tau or Aβ proteins, thus enabling them to exert clearance functionality (Wang et al., 2017). However, microglia-derived exosomes have also been shown to significantly increase Aβ neurotoxicity (Guo et al., 2021). Microglia were shown to spread tau via the secretion of exosomes and by inhibiting exosome synthesis, thus significantly reducing the propagation of tau in vitro and in vivo (Asai et al., 2015). A previous study reported that exosomes secreted from sick cells may have detrimental effects on neurogenesis and promote disease progression in AD. Exosomes derived from the conditioned medium of HEK293-APP Swe/Ind cells were injected into the hippocampal DG region; subsequent analysis revealed high neurotoxicity and impaired neurogenesis in the hippocampus of AD mice (Zheng et al., 2017).

In contrast, a greater number of NSCs in the hippocampus has been linked to reduced rates of cognitive decline in animal models of AD. According to a recent study, exosomes secreted from NSCs were enriched with a particular set of miRNAs that protected synapses from Aβ oligomer (Aβo)-binding and Aβo-induced LTP inhibition, thus protecting AD mice from subsequent memory deficits. Exosomes derived from NSCs were also found to contain a variety of proteins with functions that were positively related to neuroprotection, neurite outgrowth-promoting activity and neurogenesis. Exosomes have also been shown to reduce nerve injury in different regions of the brain and regulate inflammatory responses to promote neurogenesis in mice models of AD (Micci et al., 2019). Exosomes released from mesenchymal stem cells have also been demonstrated to ameliorate cognitive impairment and boost neuroplasticity. Exosomes produced from mesenchymal stem cells have been shown to promote neurogenesis in the SVZ and reduced Aβ_1–42-induced cognitive impairment; this finding supports the development of cell-free therapeutic strategies for AD (Reza-Zaldivar et al., 2019).

The regulation of neurogenesis by exosomes in HD

HD, a form of gene dynamic mutation illness or polyglutamine repeat disease, is a dominant hereditary neurodegenerative disease characterized by involuntary movements, mental problems, and gradual dementia. Based on current epidemiological research, the estimated prevalence of this inherited neurological disorder is approximately 1 in 20,000 to 25,000 individuals. (Pascu et al., 2015). The cause of this disease is a CAG repeat expansion on the huntingtin (Htt) gene on chromosome 4. In the brain, this expansion causes the translation of the mutant Htt protein (mHtt) which results in the generation of intracellular mHTT aggregates (inclusions) (Reindl et al., 2019). These inclusions have been linked to a series of harmful events that cause gradual changes in the structure and function of the brain.

Research has also shown that the expression of mHtt causes delayed striatal NPC development, thus resulting in changes in the striatal circuits (Lebouc et al., 2020). Alterations in striatal development caused by reduced Htt expression and/or mHtt overexpression may make striatal neurons more susceptible to cell death, thus resulting in neurodegeneration and motor abnormalities (Fu et al., 2018). With increased cell proliferation and the loss of mature neurons, adult neurogenesis appears to be reduced in the striatum of HD patients, thus indicating that neurogenesis may be initiated but limited during the maturation stage (Khuu et al., 2019). Research findings related to neurogenesis in human brains raise the prospect of endogenous neural repair for the treatment of patients with HD. A previous study indicated a statistically significant increase in cell proliferation in the subependymal layer adjacent to the caudate nucleus in response to neurodegeneration in this region in HD (Curtis et al., 2003). A previous study showed that increased severity of the pathology and the quantity of CAG repeats in the HD gene led to an increase in the level of cellular proliferation in a group of patients with HD. The most significant finding in this previous study was the proliferation of nuclear antigen positive cells co-localized with βIII-tubulin or GFAP, thus indicating that neurons and astrocytes are produced in the subependymal layer of patients with HD (Parent et al., 2002).

Exosomes are actively involved in the transmission of mHTT proteins and RNA between cells (Jeon et al., 2016). In a previous study, exosomes isolated from fibroblasts derived from HD patients were injected into the ventricles of newborn mice; this led to the development of clinical symptoms associated with HD (Jeon et al., 2016). Another study used an in vitro model of HD to demonstrate that polyglutamine and toxic CAG-repeat trinucleotide were transferred via extracellular vesicles (Zhang et al., 2016a). On the other hand, a recent study showed that exosomes derived from astrocytes reduced the density of mHTT aggregates in HD (Hong et al., 2017). In another cellular HD model, exosomes derived from adipose-derived stem cells (ASC-exo) were found to reduce mHTT aggregates and reverse mitochondrial dysfunction, thus reducing the extent of apoptosis in neurons (Lee et al., 2016). These authors found that ASC-exo considerably reduced mHtt aggregates in R6/2 mice-derived neuronal cells. Mechanically, ASC-exo upregulated PGC-1, phosphorylated CREB, and reduced aberrant levels of apoptotic proteins. ASC-exo also reduced mitochondrial dysfunction and apoptosis in a cellular model of HD as demonstrated by MitoSOX Red, JC-1, and cell viability assays. The regulatory function of exosomes isolated from different resource cells also indicated that exosome cargoes in the neurogenic niche could represent an important regulator of neurogenesis in HD patients, although this hypothesis needs to be validated by further research.

In addition to their role in regulating the progression of HD, exosomes are also thought to be a promising delivery tool for treating HD by improving endogenous neurogenesis. In the SVZ region, miR-124 was shown to induce adult neurogenesis and regulate the cell cycle in striatal neurons (Cheng et al., 2009). The delivery of miR-124 was proven to be a practical method to stimulate neuronal regeneration; this was beneficial because the striatum of HD patients exhibit neurogenic damage, thus leading to brain atrophy. In animal models of HD, exosome-based miR-124 delivery has been proven to represent a feasible method for inducing nerve regeneration (Lee et al., 2017). These results indicate that exosomes play an important role in cell communication in HD. However, the direct connection between neurogenesis and various exosomes in HD has yet to be fully elucidated. Further research is needed before exosomes can be used clinically to promote neurogenesis in HD.

Regulation of neurogenesis by exosomes in ALS

ALS, a chronic neurodegenerative disorder, usually leads to the dysfunction of lower motor neurons (the cranial nerve nuclei and anterior horn cells of the spinal cord), upper motor neurons (in the brain, brainstem, and spinal cord), and their innervated trunk, limbs and craniofacial muscles. The signs and symptoms of ALS vary greatly from person to person and depend on which neurons are affected (van Es et al., 2017). Most ALS cases (80–90%) are sporadic (sALS) while 10–20% of cases are familial (fALS) (Renton et al., 2014). Excitotoxicity and intracellular protein aggregation are thought to be mostly caused by mutations in the SOD1 gene. Other genetic factors linked to sALS and fALS include mutations in TDP43 (TAR-DNA-binding protein 43) and FUS (sarcoma fusion protein) which impair RNA processing and cause proteins to aggregate (Wu et al., 2019). The most frequent genetic cause of ALS has recently been identified as hexanucleotide repeat expansion in C9orf72, which encodes proteins that disrupt RNA and protein homeostasis (Deng et al., 2011). The ability of NPC to proliferate can be irreversibly altered by changes in the neurogenic niche of SOD1 (G93A) transgenic mice, an in vivo model of ALS (Lee et al., 2012). By using transgenic mice models, the temporal response of NPCs to motor neuron degeneration in the spinal cord and brain has been gradually revealed. Researchers often use brdU incorporation is used to identify proliferation of NPCs in the ependymal zone (EZ) around the central canal. Following treatment, NPCs lost their ability to proliferate after they migrated away from the EZ. NPCs in the EZ initially moved towards the dorsal horn; during the progression of ALS, they subsequently moved toward the region of the ventral horn where motor neurons began to deteriorate (Chi et al., 2006).

After studying the autopsies of patients with ALS, Galán et al. (2017) observed a statistically significant increase of NPC proliferation in the SVZ of all patients with ALS; this was also directly correlated with the levels of pTDP-43, a pathological hallmark for most cases of ALS, in the SVZ region. In contrast, all patients showed a reduction in the proliferation of NPCs in the SGZ region (Galán et al., 2017). Based on these findings, studies focused on the distribution of endogenous adult NPCs in an ALS mice model should be carried out during the onset of disease and during progression in order to provide fundamental guidelines for regenerative therapy in ALS by increasing de novo neurogenesis.

With regards to the mechanisms underlying neurodegeneration in ALS, numerous molecular mechanisms have been proposed, including mitochondrial failure, axonal transport, toxic protein aggregation, defective protein degradation, excitotoxicity, reduced neurotrophic support, oxidative stress, inflammation, and deficiencies of RNA metabolism. Exosomes have recently been shown to participate in the degenerative process of ALS. For example, exosomes can facilitate the transmission of mutant or misfolded SOD1 protein across cells in familial ALS (fALS) (Qualls et al., 2013). In astrocytes, mutant SOD1 causes dysfunction of the protein secretory system and exosome biogenesis. These exosomes then transmit mutant SOD1 to neurons in the spinal cord and cause motor neuron death in a selective manner (Basso et al., 2013). Studies have also indicated that misfolded SOD1 protein, produced in wild-type or mutant cells overexpressing SOD1, is associated with the secretion of exosome-like vesicles. Furthermore, this misfolded SOD1 protein can be transferred across cells in an EV-dependent manner (Grad et al., 2014).

In addition to misfolded SOD1 protein, another pathological marker, TPD-43, has also been found in exosomes derived from the CNS cells of ALS patients or animal models. Previous research showed that exosomal TDP43 oligomers were taken up preferentially by HEK-293 cells and exerted greater levels of toxicity than free TDP (Feiler et al., 2015). It has also been demonstrated that exosomal TDP-43 can activate peripheral monocytes and regulates the release of pro-inflammatory cytokines by monocytes (Zondler et al., 2017). Exosomes from hSOD1-G93A-transfected NSC-34 cells were previously loaded with miR-124 and shown to modulate the phenotype of microglia in a model of ALS (Pinto et al., 2017). Other studies have indicated that the inhibition of endogenous exosome biogenesis by GW4849, a pharmacological inhibitor, exacerbated the progression of disease in human TDP-43A315T transgenic mice. This study also indicated that exosomal signaling be beneficial for cell-to-cell communication at disease onset. Exosomal signaling may alter neuroinflammation in the neurogenic niche and significantly impact the regenerative properties of neurons in neurodegenerative diseases such as ALS. Additional regional specific or cell-type specific inhibition of exosome signaling will be needed to fully elucidate the relative contributions of exosomes in the regulation of neurogenesis in ALS.

Limitations

This paper has some limitations that need to be considered. First, we only reviewed the most common neurodegenerative diseases; we did not include acute CNS injuries such as stroke and brain disorders related to viral infection such as HIV associated neurological disorder. Second, we do not provide a comprehensive overview of the biogenesis of exosomes. Third, exosome cargoes are strictly dependent on the status of the parental cells, making these biological entities critical for the transmission of both physiological and pathological signals. We primarily focused on the pathological functions of exosomes in various diseases instead of health conditions. Exosomes derived from the cells in the neurogenic niches has also been demonstrated to play essential role in regulating adult neurogenesis. Exosomal contents such as miRNAs, proteins as well as lipids could be further caterorized to disccuss their involvement in the process of neurogenesis. Exosomes derived from the cells in the neurogenic niche has also been proved to play essential role in regulating regular adult neurogenesis. Exosomal contents such as miRNAs, proteins as well as lipids could be discussed in categories to summarize their involvement in each stage of adult neurogenesis in future.

Conclusions and Prospects

In this study, we reviewed dysfunctional patterns of neurogenesis in several neurodegenerative diseases, including PD, AD, HD and ALS. We also outlined the role of exosomes in neurogenesis and brain impairment in disease stated, as summarized in Figure 3. Exosomes can easily move across the blood-brain barrier and therefore represent an important tool for cellular communication within a neurogenic niche to modulate the process of neurogenesis. Emerging studies have also indicated that exosomes can serve as potent therapeutic carriers since they offer low immunogenicity, high stability, and both innate and acquired targetability. The transmission of exosomes between various cells has been shown to modulate the fate of NSCs within a neurogenic niche, although this form of regulation can be either beneficial or harmful; the functional effects are dependent on the cargo loaded inside the exosomes and the stage of disease (Table 1).

Figure 3.

Exosomes are secreted by glial cells and have a significant impact on the central nervous system.

Exosomes can stimulate the growth and development of synapses, neurons, and other cell types by transmitting various neurotropic factors, miRNAs, and bioactive proteins that protect neurons. Exosomes produced by glial cells also play roles in the development of neurodegenerative illnesses, including Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. For instance, exosomes are secreted to transfer inflammatory substances to stimulate immunological responses and transmit pathogenic proteins. Created using Adobe Illustrator. Aβ: Amyloid-beta; BDNF: brain-derived neurotrophic factor; iPSC: induced pluripotent stem cell; mHTT: mutant huntingtin protein; miRNAs: microRNAs; MSC: mesenchymal stem cell; SOD1: superoxide dismutase 1; α-syn: α-synuclein.

Table 1.

Exosomal protein as biomarkers for neurodegenerative disorders

Diseases	Exosome sources	Biomarkers	References
AD	Plasma, CSF	Aβ42, pT181-tau, pS396-tau, total-tau	Tapiola et al., 2009; Goetzl et al., 2016; Xiao et al., 2017; Muraoka et al., 2020
PD	Blood, CSF	α-syn, DJ-1	Shi et al., 2014; Zhao et al., 2018
HD	CSF	mHTT, Nf1, NSE	Ciancarelli et al., 2014; Wild et al., 2015; Byrne et al., 2017
ALS	Plasma, CSF, serum	TDP-43, Nfl, p-Nfh, SOD1	Kasai et al., 2009; Noto et al., 2011; Boylan et al., 2013

AD: Alzheimer’s disease; ALS: amyotrophic lateral sclerosis; Aβ₄₂: amyloid-beta 42; CSF: cerebrospinal fluid; HD: Huntington’s disease; mHTT: mutant huntingtin protein; NfL: neurofilament light chain; PD: Parkinson’s disease; p-NfH: phosphorylated neurofilament heavy chain; pS396-Tau: Tau phosphorylated at S396; pT181-Tau: Tau phosphorylated at T181; SOD1: superoxide dismutase 1; TDP-43: transactive response DNA-bindingprotein 43 kDa; α-syn: α-synuclein.

The protein oligomers in exosomal cargoes usually develop greater levels of aggregation than free oligomers; this leads to the preferential internalization of exosome-loaded oligomers and greater levels of toxicity than exosome-free oligomers. The uptake of intact exosomes by target cells involves multiple mechanisms, including caveolin-mediated endocytosis and cell membrane fusion. In neurodegenerative diseases, the phagocytosis of glial cells leads to the activation of microglia; in addition, astrocytes become actively involved in the regulation of exosomal cargo in the brain. Oligomers in a diseased brain can frequently be internalized; this can exert impact on genetic and non-genetic material in the cargo of glial cells, at least to some extent, thus leading to changes in the regulatory ability of glial exosomes within the niche (Tables 1 and 2). The role of exosomes in the pathogenesis of neurodegenerative diseases and the influence of glial exosomes on neurogenesis have yet to be fully elucidated. Further studies are needed to carefully investigate exosomal cargoes both in vitro and in vivo and determine the specific role of exosomes within the neurogenic niche, especially with regards to disease onset and progression.

Table 2.

Distinct non-genomic cargoes and effects identified in exosomes from different cells in the brain

Non-genomic cargo	Cell types	Effects	References
α-syn	Microglia	Pathological protein aggregation promotes Parkinson’s disease development	Zhao and Yang, 2021
Aβ	Microglia, astrocyte	Pathological protein aggregation promotes Alzheimer’s disease development	Kim et al., 2020; Kaur et al., 2021; Zhao et al., 2022
mHTT	Astrocyte	Pathological protein aggregation promotes Huntington’s disease development	Hong et al., 2017
SOD1	Astrocyte	Transfer to spinal cord neurons and induce selective motor neuron death	Silverman et al., 2019
TDP-43	Astrocyte	Transfer to spinal cord neurons and induce selective motor neuron death	Iguchi et al., 2016; Maguire et al., 2019
IL-1β	Microglia	Induce and propagate inflammatory reactions throughout the brain	Jiang et al., 2021; Liu et al., 2022; Xian et al., 2022
BDNF	MSCs	Reduce amyloid-beta induced cognitive impairment	Xu et al., 2020; Ahn et al., 2021
Lipids	Oligodendrocyte	Neuroprotection and long-term axonal maintenance	Reiter and Bongarzone, 2020
44Q	Fibrocyte	Neuroprotection	Ren et al., 2009
Hsp70	Astrocyte	Promote neuronal survival	Attili et al., 2020
Wnt5a	BMSC	Inhibit the release of inflammatory factors and reduce M1 polarization in microglia	Xu et al., 2022
caspase-1	Microglia	lL-1β processing enzyme	Zhou et al., 2022
synapsin-1	Astrocyte	Neuroprotection	Upadhya et al., 2020
Myelin proteins	Oligodendrocyte	Neuroprotection and long-term axonal maintenance	Aneesh et al., 2021

Aβ: Amyloid-beta; BDNF: brain-derived neurotrophic factor; BMSC: bone marrow mesenchymal stem cell; Hsp70: heat shock protein 70; IL-1β: lnterleukin-1 beta; mHTT: mutant huntingtin protein; SOD1: superoxide dismutase 1; TDP-43: transactive response DNA-bindingprotein 43 kDa; Wnt5a: Wingless-related MMTV integration site 5A; α-syn: α-synuclein.

An increasing number of studies have also demonstrated the beneficial properties of exogenous cell-derived exosomes in regenerative medicine (Tables 2 and 3). MSC-derived exosomes have already been directly injected into the neurogenic niche to promote neurogenesis in several models of neurodegenerative disease models. Using cargo in endogenous or exogenous cell-derived exosomes to control adult neurogenesis has significant potential for the treatment and prognosis of neurodegenerative diseases. However, we still need to investigate the specific contents of exosomal cargo, along with associated regulatory mechanisms, if we are to fully understand the functional role of exosomes within the neurogenic niche. Furthermore, investigating the exosomal cargoes released by numerous niche cells that can impair neurogenesis could also identify useful targets to rescue the impaired environment provided by the niche and promote neurogenesis in neurodegenerative diseases. Before exosomes can be translated from the laboratory to the clinic, it is important to consider targeted delivery in more detail. This is because the delivery of exosomes to specific types of cells in the neurogenic niche could increase the local concentration of therapeutics and minimize side effects. Finally, we believe that future collaboration between researchers from various disciplines, such as clinicians, neurobiologists, technology authorities, and computer experts, could result in significant advancements in the future development of exosome-based therapy to promote neurogenesis in neurodegenerative diseases.

Table 3.

Distinct genomic cargoes and effects identified in exosomes from central nervous system cells in neurodegenerative disorders

Genomic cargo	Cell types	Effects	References
miR-1	NSC	Increases hippocampal neurogenesis	Micci et al., 2019
miR-17-92	MSC	Reducing binding of Aβ to synapses increases synaptic resistance protectingtheprotecting the hippocampus from Aβ-induced LTP inhibition	Micci et al., 2019
miR-124	NSC	lnduced adult neurogenesis in the subventricular zone and regulated cell cyclein striatal neurons	Cheng et al., 2009
miR-124	NSC-34	Drives changes in microglial phenotype and suppresses inflammatory responses	Pinto et al., 2017
miR-322	NSC	Reducing binding of Aβ to synapses increases synaptic resistance protectingtheprotecting the hippocampus from Aβ-induced LTP inhibition	Micci et al., 2019

Aβ: Amyloid-beta; LTP: long-term potentiation; MSC: mesenchymal stem cell; NSC: neural stem cell.