Ischemic stroke is a life-threatening cerebral vascular disease and accounts for high disability and mortality worldwide. Currently, no efficient therapeutic strategies are available for promoting neurological recovery in clinical practice, except rehabilitation.
1. Introduction
Ischemic stroke is the leading cause of long-term disability and death in adults worldwide
[1][2]. Rehabilitation following ischemic stroke is related to a complex set of courses
[3]. Most survivors suffer from a series of neurological dysfunctions and experience modest functional recovery after ischemic stroke. Patients are often unable to be independent in their daily lives, which seriously affects their families socially and economically. Presently, numerous efforts have been devoted to investigating the pathogenesis of this condition and to discovering potential drugs for ischemic stroke by preclinical and clinical studies
[4][5][6]. The only useful interventions are thrombolytic therapy with recombinant tissue plasminogen activator (r-tPA) and endovascular thrombectomy. Nevertheless, both approaches have an extremely narrow treatment window (generally within 4.5 h) after the onset of stroke, and few patients benefit from these treatments
[7][8][9]. In addition, though various drugs showed a positive impact in pre-clinical studies, none of them has appeared to be able to restore the neurological function
[10][11][12][13]. Reliable and effective therapeutic approaches are urgently needed for ischemic stroke patients.
Previous preclinical and early-phase clinical data have confirmed that cell therapy is a safe and promising option for the recovery of neurological function following ischemic stroke
[14][15][16][17]. An increasing number of studies suggest that the therapeutic effect of stem cells is mainly mediated by exosomes released from the administered cells
[18][19]. Exosomes act as messengers to mediate intercellular communication by delivering biological material, including microRNA and proteins, which plays indispensable roles in physiological and pathological processes
[20][21]. Compared with their parent cells, exosomes have a nanoscale size and a lower expression of membrane-bound proteins, which leads to minimal immune response and toxicity in non-immunosuppressed models
[22][23][24]. Furthermore, exosomes are stable in the circulation and have the ability to cross the blood–brain barrier (BBB)
[25]. These unique characteristics have brought more attention to exosomes.
2. Characteristics of Exosomes
Exosomes represent a subspecies of extracellular vesicles (EVs) with structural size ranging from 30 to 150 nm, released from most cells in all living systems. They exist in various body fluids, such as cerebral spinal fluid, blood, saliva, and urine
[26][27][28][29]. Exosomes are initiated by the invagination of the endosomal membrane (
Figure 1). Intraluminal vesicles (ILVs) are generated by the inward budding of the endosomal membrane, and early endosomes mature into multivesicular bodies (MVBs). MVBs bind to lysosomes, which results in the degradation of their contents. Additionally, MVBs may fuse with the plasma membrane, leading to the secretion of exosomes
[30][31]. The biogenesis of exosomes is strictly regulated by many cell proteins, including Alix, Rab27a, soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), and cortactin
[32][33][34].
Figure 1. Biogenesis and Secretion Processes of Exosomes and Exosomal microRNAs.
Exosomes are uniformed spheroids with a bilayered lipid membrane. The membrane is rich in cholesterol, sphingomyelin, ceramide, and surface markers from the parent cell, including tetraspanin proteins (CD63, CD81, and CD9), flotillin 1, Alix, and tumor susceptibility gene 101 protein
[35][36]. Exosomes carry biologically active substances (proteins, lipids, DNAs, RNAs, and microRNAs), which can mediate cellular communication and modulate a series of physiological processes
[37]. Exosome proteins are implicated in antigen presentation, immune reaction, cell binding, and catalytic activity. In addition, proteins (such as brain-derived neurotrophic factor, Zeb2/Axin2) of exosomes participate in brain repair, involving neurogenesis, antiapoptotic processes, and synaptic transmission
[38][39]. Of the exosome cargo, microRNAs appear to participate in multiple biological processes
[40]. For instance, miRNA-143 is involved in angiogenesis
[41], while miR-17-92 cluster and miR-26a are related to neurogenesis and axonal growth signals
[42][43].
MicroRNAs are short single-stranded noncoding RNAs and generally consist of 20–25 nucleotides. MicroRNAs are transcribed by RNA polymerase II as primary microRNAs (pri-miRNAs) (
Figure 1). The mature microRNA sequence is embedded in the hairpin of pri-miRNA. Following transcription, pri-miRNAs are cleaved into ~65 nt precursor microRNAs (pre-miRNAs) by microprocessor. Pre-miRNAs are transferred into the cytoplasm and processed into double-stranded RNAs, which are 20–25 nt long
[44][45]. Ultimately, one strand integrates into a silencing complex and is loaded into MVBs. Along with exosomes, mature microRNAs are released into recipient cells, which are involved in regulating post-transcriptional gene expression and modulating a variety of cellular and molecular pathways
[46][47].
Emerging studies have demonstrated that exosomes are involved in the modulation of physiological and pathological processes after ischemic stroke and contribute to brain remodeling by transferring of their cargo. Hence, exosomes have been considered as promising biomarkers for the early diagnosis and prognosis of ischemic stroke and as perspective drugs for the treatment of ischemic stroke
[48][49].
3. Roles of Exosomes in Ischemic Stroke
3.1. Exosomes and Ischemic Stroke Diagnosis
In the central nervous system, exosomes derived from brain cells play significant roles in regulating normal physiological process and responding to acute brain injury
[50]. Brain cells, including neurons, microglia, oligodendrocytes, astrocytes, endothelial cells, and pericytes, communicate with each other via their exosomes and exosomal cargos to regulate brain functions, from antioxidation to BBB integrity maintenance and synaptic function
[51][52]. Following injury, exosomes are generated by brain cells and evoke diverse responses. Some exosomes seem to have beneficial effects in neuroprotection and neurological recovery. However, some exosomes also have adverse impacts involving neurodegeneration and neuroinflammation
[53][54]. Moreover, these exosomes can pass through the BBB and circulate in the peripheral blood and cerebrospinal fluid and could be excellent noninvasive biomarkers for ischemic stroke diagnosis and prognosis
[55][56]. Recent studies have detected many components in circulating exosomes which could serve as biomarkers for ischemic stroke, particularly, microRNAs (
Table 1)
[57][58].
A clinical study indicated that the expression of serum exosomal miR-9 and miR-124 was increased in patients with ischemic stroke and was also positively associated with infarct volume, serum IL-6 concentration, and National Institutes of Health Stroke Scale (NIHSS) scores
[57]. These two exosomal microRNAs are considerable biomarkers for diagnosing ischemic stroke and evaluating the degree of ischemic injury
[57][59]. Other studies found that circulating exosomal miR-223 and miR-134 were obviously upregulated in acute ischemic stroke patients, strongly associated with NIHSS scores and the expression of IL-6 and high-sensitivity C-reactive protein, and correlated with the occurrence, severity, and worse prognosis of acute ischemic stroke
[58][60].
Table 1. Exosomal microRNAs as Biomarkers in the Diagnosis of Ischemic Stroke.
| microRNAs |
Expression in IS |
Sources |
Models |
Outcomes |
References |
| miR-9 |
upregulation |
serum |
Human |
NIHSS score, infarct volume, serum IL-6 |
[57] |
| miR-124 |
upregulation |
serum |
Human |
NIHSS score, infarct volume, serum IL-6 |
[57] |
| miR-223 |
upregulation |
serum |
Human |
NIHSS score, infarct volume, 3-month mRS, stroke occurrence |
[58] |
| miR-134 |
upregulation |
serum |
Human |
NIHSS score, infarct volume, serum IL-6, hs-CRP |
[60] |
| miR-422a |
upregulation in acute phase downregulation in subacute phase |
plasma |
Human |
different stages of IS |
[61] |
| miR-125-2-3p |
downregulation |
plasma |
Human |
different stages of IS |
[61] |
| miR-21-5p |
upregulation in subacute phase upregulation in recovery phase |
plasma |
Human |
different stages of IS |
[62] |
| miR-30a-5p |
upregulation in hyperacute phase downregulation in acute phase |
plasma |
Human |
different stages of IS |
[62] |
| miR-17-5p |
upregulation |
serum |
Human |
subtypes of stroke |
[63] |
| miR-20b-5p |
upregulation |
serum |
Human |
subtypes of stroke |
[63] |
| miR-27b-3p |
upregulation |
serum |
Human |
subtypes of stroke |
[63] |
| miR-93-5p |
upregulation |
serum |
Human |
subtypes of stroke |
[63] |
| miR-15a |
downregulation |
serum |
Human |
subgroups of stroke |
[64] |
| miR-100 |
downregulation |
serum |
Human |
subgroups of stroke |
[64] |
| miR-339 |
downregulation |
serum |
Human |
subgroups of stroke |
[64] |
| miR-424 |
downregulation |
serum |
Human |
subgroups of stroke |
[64] |
| miR-122-5p |
downregulation |
plasma |
Rat |
different stages of IS |
[65] |
| miR-300-3p |
upregulation |
plasma |
Rat |
different stages of IS |
[65] |
| miR-126 |
downregulation |
serum |
Rat |
different stages of IS |
[59] |
3.2. Exosomes and Ischemic Stroke Treatment
Multiple studies demonstrated that cell-based therapy is an excellent method to promote functional outcomes after ischemic stroke, especially if based on mesenchymal stem cells (MSCs)
[66][67][68][69]. Exosomes play a significant role in the paracrine effects of stem cells
[70][71]. Exosomes from stem cells show low immunogenicity, low tumorigenicity, high transportation efficiency, innate stability, and the capacity to cross the BBB
[72][73][74]. They have demonstrated beneficial effects by improving functional recovery after ischemic stroke, because of their ability to enhance brain plasticity
[75][76].
Clinical evaluation of exosome therapeutics remains extremely limited, but promising efficacy has been observed in animal ischemic stroke models
[77]. Doeppner et al. showed that exosomes from bone marrow MSCs (BMMSCs) efficiently reduced peripheral immunosuppression, enhanced neurovascular regeneration, and improved the motor function 4 weeks after ischemia
[78]. MCAO (middle cerebral artery occlusion) rats achieved better results after with intravenous infusion of exosomes in foot fault and modified neurologic severity scores, compared to the PBS group
[75]. Exosome treatment post stroke promoted neurite remodeling, angiogenesis, and neurogenesis
[75]. Therapy based on exosomes from adipose-derived MSCs (ADMSCs) could reduce the brain infarct zone, improve the recovery of neurological function, and enhance fiber tract integrity and white matter repair in rats after stroke
[79][80].
Numerous research studies illustrated that exosomes modulate the recipient cells and the rehabilitation process after stroke primarily via microRNAs (
Table 2). Xin et al. revealed that exosomes mediated miR-133b transfer to astrocytes and neurons, subsequently enhancing neurite outgrowth and promoting functional recovery after ischemic injury
[81][82][83]. Similarly, exosomes enriched with miR-17-92 cluster showed robust effects on neurological function rehabilitation and neural plasticity by modulating the PTEN/Akt/mTOR signaling pathway
[84][85]. Another study found that miR-138-5p-enriched exosomes alleviated neurological impairment by accelerating the proliferation of astrocytes and suppressing inflammation by targeting lipocalin 2 in ischemic stroke mice
[86]. Furthermore, miR-30d-5p- and miR-223-3p-enhanced exosomes could attenuate cerebral ischemia injury by inhibiting M1 polarization of microglia
[87][88]. Exosomes with miR-1906 overexpression downregulated the TLR4 level and enhanced neuroprotection in ischemic mice
[89]. miR-132-3p promoted the beneficial effects of exosomes, reducing cerebral vascular ROS production, BBB dysfunction, and brain injury
[90]. In addition, miR-21-3p, miR-134, miRNA-184, and miRNA-210 in exosomes were also essential for the prevention of ischemic injury
[91][92][93]. Therefore, tailored exosomes with an optimal beneficial microRNA content may maximize their therapeutic potential for ischemic stroke or other neurological disorders. These emerging data highlight the importance of exosomes and their cargos, in particular miRNAs, for brain-remodeling processes.
Table 2. Exosomal microRNAs have been Used to Treat Ischemic Stroke.
| microRNAs |
Models |
Sources |
Proposed Effects |
Involved Pathway |
References |
| miR-133b |
MCAO-rat |
MSC |
Neural remodeling |
CTGF |
[81][82][83] |
| miR-17-92 cluster |
MCAO-rat |
MSC |
Neural remodeling |
PTEN/Akt/mTOR pathway |
[85] |
| miR-138-5p |
MCAO-mouse OGD-astrocyte |
MSC |
Anti-inflammation Anti-apoptosis |
Lipocalin 2 |
[86] |
| miR-30d-5p |
MCAO-rat OGD-microglia |
MSC |
Anti-inflammation Anti-apoptosis |
Beclin-1/Atg5 |
[87] |
| miR-223-3p |
MCAO-rat OGD-microglia |
MSC |
Anti-inflammation |
CysLT2R-ERK1/2 |
[88][94] |
| miR-1906 |
MCAO-mouse OGD-neuron |
MSC |
Anti-inflammation |
TLR4 |
[89] |
| miR-132-3p |
MCAO-mouse endothelial cell |
MSC |
BBB protection Reduce vascular ROS |
PI3K/Akt/eNOS pathway |
[90] |
| miR-21-3p |
MCAO-rat |
MSC |
BBB protection Anti-inflammation Anti-apoptosis |
MAT2B |
[91] |
| miR-134 |
OGD-oligodendrocyte |
MSC |
Anti-apoptosis |
Caspase-8 |
[92] |
| miR-184 |
MCAO-rat |
MSC |
Neurogenesis Angiogenesis |
---- |
[93] |
| miR-210 |
MCAO-rat |
MSC |
Neurogenesis Angiogenesis |
ephrin-A3 |
[93] |
| miR-126 |
MCAO-mouse |
EPC |
Neurogenesis Angiogenesis Anti-apoptosis |
Caspase-3VEGFR2 |
[95][96] |
| miR-181b-5p |
OGD-endothelial cell |
MSC |
Angiogenesis |
TTRPM7 |
[97] |
| miR-132 |
zebrafish larvaeendothelial cell |
Neuron |
Angiogenesis |
Cdh5/eEF2K |
[52] |
| miR-124 |
Photothrombosis mouse |
MSC |
Neurogenesis |
GLI3 STAT3 |
[98] |
MCAO-mouse OGD-neuron |
M2 microglia |
Anti-apoptosis |
USP14 |
[99] |
| miR-137 |
MCAO-mouse OGD-neuron |
Microglia |
Anti-apoptosis |
Notch1 |
[100] |
| miR-22-3p |
MCAO-rat OGD-neuron |
MSC |
Anti-apoptosis |
KDM6B/BMP2/BMF axis |
[101] |
| miR-34c |
MCAO-rat OGD-neuroblastoma cells |
Astrocyte |
Anti-inflammation Anti-apoptosis |
TLR7 and NFκB/MAPK pathways |
[102] |
| miR-146a-5p |
MCAO-mouse OGD-microglia |
MSC |
Anti-inflammation |
IRAK1/TRAF6 pathway |
[103] |
This entry is adapted from the peer-reviewed paper 10.3390/biom12010115