Neurodegenerative diseases are chronic, latent, progressive disorders that occur in the central nervous system (CNS). They are characterized by the loss of neuronal structure and function
[88][46] lead to Information transmission disorder for neuron-to-neuron. Although great progress has been made in neuroscience research, there are still huge shortcomings. The main and most important is the lack of thorough understanding of its specific pathological mechanism. Moreover, the diseases of the nervous system are more complicated and multifactorial, which makes it more difficult to study
[6][47]. Due to the extremely weak self-regeneration of neurons, the dysfunction after central nervous system injury often accompanies patients for life, so it has become a difficult problem to be broken through in clinical treatment. In the treatment of central nervous system injury, MSCs-Exo has been proved to have definite therapeutic effects of promoting neurovascular regeneration, regulating inflammatory environment and repairing nerve myelin sheath. Many researchers have confirmed that stem-cell-derived exosomes have neuroprotection and neurotrophy effects
[36,89][22][48] (
Figure 2). Furthermore, the small size of MSCs-Exo makes itself cross the blood–brain barrier, reaching the brain or spinal cord tissue
[36][22].
Stroke is one of the leading causes of death
[96][49]. Some cell-based therapies have been deliberated to promote stroke treatment in preclinical and clinical trials
[97][50], and the secretion of stem cells, such as exosomes, has also shown to be promising in various preclinical models of stroke recovery
[98][51]. Xin et al. found that in rat models of stoke, administration of MSCs-Exo has been shown to enhance neurogenesis and angiogenesis and improve brain function recovery
[95][52]. In amyotrophic lateral sclerosis (ALS), related studies have found that misfolded SOD1 protein can be transferred from cell to cell through exosomes dependence and exosomes independence, make progating diaeases
[99][53]. In addition, another study indicated that TDP-43 was also associated with exosomes. Exosomes are an important pathway for THE transfer of TDP-43 aggregates
[100][54]. Both SOD1 protein and TDP-43 protein are important pathological features of ALS. Exosomes also act through anti-apoptosis and anti-necrosis mechanisms (activating cell survival PI3K-B-cell lymphoma-2 (Bcl-2) pathway). Additionally, endogenous neuronal survival factors play an important role in the treatment of ALS by enhancing the receptor cells
[101][55]. Riazifar and their colleagues assessed the effect of MSCs-Exo in treating multiple sclerosis and found that intravenous administration of MSCs-Exo can decrease neuroinflammation and reduce demyelination
[65][23]. Additionally, in multiple sclerosis it often presents as chronic inflammation. MSCs-exo can also play an important role in this regard. It can regulate the activation of microglia by and by inhibiting the release of pro-inflammatory cytokines, greatly reducing the amount in the plasma to reduce inflammatory infiltration.
A study [102] used [56]IFNγ-stimulated dendritic cell cultures
were used to release exosomes that increased myelin levels and reduced oxidative stress and promote myelin reformation after demyelination. Chen and their partners administrated human adipose mesenchymal stem cell (HaMSC)-derived exosomes (HaMSC-Exo) into a weight-drop-induced traumatic brain injury (TBI) rat model and found that HaMSC-Exo promoted functional recovery, suppressed neuroinflammation, reduced neuronal apoptosis, and increased neurogenesis in TBI rats
[80][57]. In a study to test the influence of MSCs-Exo in a large animal model of TBI, experts used MSCs-Exo to administrate female Yorkshire swine after TBI. They found that exosome-treated animals had significantly attenuated brain swelling and smaller lesion size, decreased blood-based cerebral biomarkers levels, and improved blood–brain barrier (BBB) integrity
[81][58]. The inflammatory response activated after nerve injury may cause a secondary attack on the lesion. However, MSCs-Exo significantly prevented the pro-inflammatory cytokine release while promoting an M1 to M2 phenotype polarization in microglia and thereby reducing inflammatory damage. de Godoy, Mariana A. et al. have shown that MSC-EVS can reduce the expression of ROS related fluorescence signal in AD hippocampus neurons in vitro and protect neurons from Aβ protein-induced oxidative damage
[103][59]. Recent studies have found that MSCs-Exo can significantly reduce the accumulation of β-amyloid (Aβ) protein in neurons, which confirming the therapeutic effect of MSC-EVS on the alleviation of pathological changes in Alzheimer’s disease
[104][60]. In AD mice model, experts found that intracerebroventricularly injected BM-MSCs can improve cognitive impairment by ameliorating astrocytic inflammation as well as synaptogenesis
[93][61]. In another study, MSCs-Exo was injected into APP/PS1 mice, and after a period of treatment, the ability of spatial learning and memory was significantly improved. The symptoms associated with AD were significantly improved. It was confirmed that the activation of SphK/S1P signaling pathway could reduce Aβ deposition and promote the recovery of cognitive function in AD mice
[105][62]. As neurodegenerative diseases are characterized by the intracellular or extracellular aggregation of misfolded proteins
[106][63], some experts intend to find early content changing of exosomes as biomarkers for AD/PD diagnosis. Yang et al. found that the serum exosomes-derived microRNA, miR-135a, -193b, and -384, are potential biomarkers for early AD diagnosis. In addition, MSC-EVs also has a regulatory effect on the microglial immune activated by Aβ, which can improve the neuronal survival in AD brain. Research found that MSC-EVS could inhibit microglia polarization to pro-inflammatory M1 subtype and increase the number of anti-inflammatory M2 subtype microglia in AD transgenic mice, and upregulate the expression of anti-inflammatory progenitor TGF-β and IL-10 in brain tissues
[107][64]. This immunomodulatory effect is also involved in the protective effect of MSC-EVs on AD neurons. Additionally, researchers discovered that the exosomes from AD patients might become toxicity vesicles containing toxic amyloid-beta protein
[108][65]. That also illustrates that exosomes are closely related to the occurrence of
central nervous system CNSDdisorder/disease (CNSD). Wang et al. development of exosome as a carrier for curcumin prevents neuronal death in vitro and in vivo to alleviate AD symptoms. This study provides potential clinical evidence for exosome-based drug delivery in the treatment of AD
[109][66]. Meckes Jr et al. found that 5 × FAD mice received hMSCs-Exo treatment can slow down AD pathogenesis and ameliorate inflammatory marker glial fibrillary acidic protein (GFAP) in a preclinical mouse model
[92][67]. However, the efficacy of MSCs-Exo demonstrated only from the perspective of Aβ protein may require further validation in future clinical trials. Exosome-associated miR-137 has been found to be upregulated in neurons in PD, where it plays a vital role in neuronal oxidative stress induction. MiR-137 directly targets oxidation resistance-1 (OXR1) to negatively regulate its expression, thereby inducing oxidative stress. The levels of miRNAs have also been investigated in some PD models, such as in a manganese model where 12 miRNAs were significantly increased in exosomes; these miRNAs were shown to regulate key PD pathogenesis pathways including autophagy, inflammation and protein aggregation
[110][68]. Another group reported that exosome delivery of hydrophobically modified siRNA to the brain efficiently targeted mHtt mRNA in a Huntington’s disease model, which is encouraging for the potential use of siRNAs to target α-syn in PD
[111][69]. MSC-derived exosomes proved effective at rescuing dopaminergic neurons in the 6-OHDA mouse model of PD, and they can also carry miRNAs and interact with neuronal cells to reduce neuroinflammation and promote neurogenesis in mouse PD models
[112][70]. In another study, also using a 6-OHDA mouse MODEL of PD, treatment with SHED-derived exosomes was carried out. The expression level of TH in striatum and substantia nigra was decreased, demonstrating the potential of exosomes in PD treatment
[113][71]. Whilst further investigations and clinical trials are required to confirm the benefits of therapeutic application of exosomes in PD, mounting evidence supports that the separation of exosomes from various cell types and their modification to target specific brain regions may hold therapeutic benefits for PD, among other disorders
[114][72].