Retraction Note: Mesenchymal stem cell-derived exosome miR-542-3p suppresses inflammation and prevents cerebral infarction

Cerebral infarction ranks as the second leading cause of disability and death globally, and inflammatory response of glial cells is the main cause of brain damage during cerebral infarction. Studies have shown that mesenchymal stem cells (MSCs) can secrete exosomes and contribute to cerebral disease. Here, we would explore the function of MSC-derived exosome in cerebral infarction. Microarray indicated a decrease of miR-542-3p and an increase of Toll-Like Receptor 4 (TLR4) in middle cerebral artery occlusion (MCAO) mice comparing with sham mice. And luciferase and RIP analysis indicated a binding of miR-542-3p and TLR4. Then, we injected AAV9-miR-542-3p into paracele of sham or MCAO mice. Functional analysis showed that AAV9-miR-542-3p inhibited infarction area and the number of degenerating neurons and suppressed inflammatory factors’ expression and inflammatory cell infiltration. As well, transfection of miR-542-3p mimics into HA1800 cells underwent oxygen and glucose deprivation (OGD). Similarly, overexpression of miR-542-3p alleviated OGD induced cell apoptosis, ROS, and activation of inflammation response. Moreover, miR-542-3p could be packaged into MSCs and secreted into HA1800 cells. The extractive exosome-miR-21-3p treatment relieved MCAO- or OGD-induced cerebral injury and inflammation through targeting TLR4. These results confirmed that MSC-derived exosome miR-542-3p prevented ischemia-induced glial cell inflammatory response via inhibiting TLR4. These results suggest possible therapeutic strategies for using exosome delivery of miR-542-3p to cure cerebral ischemic injury.


Background
Cerebrovascular disease is a kind of disease with acute neurological disease and common system, which has the characteristics of high fatality rate, disability rate, and recurrence rate [1]. According to the data released by the World Health Organization, stroke has become the second leading cause of death worldwide in the past 10 years from 2000 to 2012 [2]. Among them, ischemic cerebral infarction is the most common type of stroke, which occurs after cervical or cerebral blood flow is blocked [3]. Due to the active metabolism of the brain, more oxygen is needed. Once the blood flow is blocked and the perfusion cannot be restored in a short period of time, the brain cells will die due to hypoxia, which in turn induces brain dysfunction [4]. Although the function of surviving neurons can at least partially compensate for brain damage after ischemia, the repair ability of human brain neurons is limited [5]. At present, thrombolysis is the most effective method for the clinical treatment of ischemic stroke, but thrombolysis has a strict treatment time window, and the vast majority of patients have lost the opportunity of thrombolytic therapy when they are admitted to hospital [6]. Therefore, finding new therapeutic targets is of great significance for the treatment of ischemic stroke.
Inflammatory response is one of the important mechanisms of dysfunction caused by cerebral infarction [7,8]. Ischemia induces the release of pro-inflammatory cytokines (TNF α, IL-1, IL-6, MCP1) and inflammatory mediators (ROS and NOS) from glial cells and endothelial cells [9][10][11]. These cytokines recruit leukocytes and stimulate the production of adhesion molecules on leukocytes and endothelial cells, causing phagocytic activity and immune response [12]. It has been confirmed that IL-1 releases arachidonic acid and increases the number of TNFα and IL-1β, which leads to the accumulation of inflammatory cells and aggravates brain injury [13]. Therefore, the understanding of inflammatory response is very important to understand the pathogenesis and explore the treatment of hypoxic-ischemic brain injury.
Exosomes originate from cellular polyvesicles [14]. After combining with the plasma membrane, exosomes can be secreted out of cells. Almost all types of cells can secrete exosomes, which are widely distributed in body fluids, in which there are intracellular sources of lipids and RNA and other bioactive substances [15]. Exosomes can mediate intercellular communication, and it has been confirmed in recent studies that exosomes can participate in the remodeling process after cerebral infarction by mediating nucleotide and protein affecting angiogenesis and neuroglia [16,17]. At present, exosomes in the treatment of cerebral infarction is still limited to clinical research, mainly in stem cell expression of related genes and transport-related regulatory proteins. In addition, as a drug delivery carrier, therapeutic drugs are transferred into the exosomes by means of electroporation to effectively achieve the purpose of treatment [18]. When exosomes played a role in stem cell expression of related genes and transport-related regulatory proteins, the motor function of exosomerelated model mice derived from stem cell (MSCs) recovered significantly. Exosomes on the basis of MSCsmediated miR-133b can effectively improve the neurite remodeling of ischemic model mice and restore the function of model mice [19]. In addition, exosomes can also play a full role on the basis of transport-related regulatory proteins. Exosomes released by MSC have been found to contain 1927 proteins [20]. Proteomic analysis has proved that exosomes released by MSC have the effect of angiogenic paracrine, which indicates that exosomes released by MSC have the role and great potential in the treatment of ischemic tissue-related diseases [21,22].
In recent years, related studies have shown that miR-NAs play an important role in all stages of the process of cerebral ischemic injury and can aggravate or alleviate brain injury in a variety of ways [23]. Chi et al. have confirmed that overexpression of miR-134 aggravates cell death and apoptosis in vivo and in vitro [24]. When miR-134 was deficient, the level of HSPA128 protein increased in N2A cells treated with oxygen and glucose deprivation and ischemic brain tissue. MiR-134 deficiency can reduce the cerebral infarction area and nerve cell injury and improve the neurological function score in mice. Sun et al. found that overexpression of miR-124 significantly increased the infarct size of the mouse model of middle cerebral artery ischemia, while knockout of miR-124 significantly reduced the infarct size [25]. MiR-124 plays a neuroprotective role by inhibiting apoptosis in the pathological process of ischemic stroke. MiR-542-3p is mainly studied in tumors, also expressed in brain tissues [26]. And He et al. indicated that miR-542-3p might regulate pathological processes of ischemic stroke [27]. However, the role of miR-542-3p in cerebral infarction is poorly identified. Herein, we explored the effect of miR-542-3p in cerebral infarction-induced glial cell inflammation and injury and clarify whether miR-542-3p could be packaged into exosomes to treat cerebral infarction.

Animal experiments
Middle cerebral artery occlusion (MCAO) of mice used for cerebral infarction model. As previously described [28], left distal middle cerebral artery was occluded after the mice were completely anesthetized. The mice in the sham group only received vascular separation without ligation. One hundred-microliter AAV9-miR-542-3p/ AAV9-miR-NC (2.0 × 10 11 GC/ml) was injected into paracele of mice at 1 h after MCAO or sham operation. Neurological assessment used to determine neurological deficiency of mice at 3 days after MCAO operation [29]. The left hemispheres of the mice were extracted, and the brain tissue water content was detected as previously described [30]. The research protocol of this study was approved by the Animal Care and Use Committee of the Second Affiliated Hospital of Heilongjiang University of Traditional Chinese Medicine.

Cell culture and treatment
The MSCs and HA1800 cells (human glial cells) were purchased from the Science Cell Laboratory. Cell lines were cultured in DMEM (Thermo-life, USA) with 10% FBS (Thermo Fisher, USA) and 100 μL/mL penicillin and streptomycin (Beyotime, China) and placed at 37°C with 5% CO2. The HA1800 cells were plated until the cell density reached 80% confluency of dishes to transfect. MiR-542-3p mimics and Toll-Like Receptor 4 (TLR4) plasmid were constructed by Genechem (Shanghai, China) and transfected into cells with Lipofectamine 2000 (Invitrogen, Carlsbad, CA). For oxygen and glucose deprivation (OGD), HA1800 cells were seeded in Hank's medium and cultured at 37°C with 5% CO2 and 95% N2 for 2 h.

Exosome isolation and identification
Exosomes in supernatant of MSCs transfected with miR-542-3p/miR-NC were isolated, and several centrifugations were performed to purify exosomes [31]. Transmission electron microscopy (TEM) was used to identify exosomes structure. The diameter of exosome was examined by particle size analysis, and exosome marker protein CD63, CD9, and GM130 were tested by western blot.

MTT assay
HA1800 cells were plated in 96-well plates, and we used MTT assay to detect the cell viability. MTT (0.5 mg/mL; Beyotime Biotechnology, China) was added to every well after treatment and incubated for 3 h at 37°C. And 150 μL DMSO was added and incubated for 15 min. We measured the absorbance by Spectrophotometer (Tecan, Austria) at 493 nm.

ROS assay
Reactive oxygen species (ROS) detection was performed according to the procedures (Beyotime, China). Briefly, HA1800 cells were plated in 12-well plates and ROS solution was added into cells for 20 min. After fixation in 4% paraformaldehyde and PBS washing solution, the cells were incubated in DAPI for 10 min. Fluorescence was observed by fluorescence microscope.

qRT-PCR
RNA extraction was performed using trizol reagent. NanoDrop 8000 (Thermo Scientific, Waltham, MA, USA) was used to detect the concentration and purity of RNA. The single-stranded cDNAs were synthesized from 1 μg of RNA. The expression of mRNAs and miRNAs were quantified by RT-PCR with SYBR Green I (Thermo Fisher Scientific, Inc).

RIP
We used RIP assay to determine the binding between TLR4 and miR-542-3p using Magna RIP™ RNA-Binding Protein Immunoprecipitation Kit (Millipore) as previous study [32]. Briefly, HA1800 cells were transfected with biotinylated TLR4, and the mRNA level of miR-542-3p was detected using qRT-PCR.

H&E staining
The brain tissues were gathered and fixed in 4% paraformaldehyde for 24 h. Then, the fixed tissues were embedded in paraffin. Next, Paraffin slicer machine was used to cut slices (5-mm cross-sectional). H&E staining was used to evaluate pulmonary morphology. Liver sections were dewaxed with xylene and treated with ethanol at different concentrations for 5 min. Hematoxylin staining for 5 min, 5% acetic acid treatment for 1 min, water rinse. Dye with eosin for 1 min, rinse with running water. Dehydrate in 70%, 80%, 90%, 100% ethanol for 10 s, xylene for 1 min. Drizzle with neutral gum and seal.

TTC staining
TTC staining was used to evaluate the infarcted size of the brain. The brain sections were placed in the smallest vessel containing 2% TTC staining solution (G3005, Solarbio) and incubated at 37°C in darkness for 15-30 min. The color changes of the samples were observed while incubating. The excess staining solution on the tissue surface was rinsed off with PBS and photographs were taken.

Immunohistochemistry (IHC) staining
Paraffin sections of brain tissues from different groups were dewaxing to water in xylene and descending series of ethanol. We penetrated sections using 0.5% Triton X-100. After 3 times wash, we blocked sections with 50% goat serum. Then, sections were incubated with VEGFA antibody (ab51745, Abcam) overnight. We incubated the sections using secondary antibody. The sections were photographed by light scope under an IX73 fluorescence microscope (Olympus, Valley, PA).

Fluoro-Jade C (FJC) staining
FJC staining was used to determine the degenerating neurons in brain tissues. Brain sections were stained using Fluoro-Jade® C kit (TR-100-FJ, Biosensis) according to the protocol and previous study [33].

TUNEL
We used the in situ Cell Death Detection Kit (TUNEL fluorescence FITC kit, Roche, Germany) to detect apoptotic. We used DAPI to stain nuclei. We used IX73 fluorescence microscope (Olympus, Valley, PA) to analyze fluorescence staining. We used Image-J to count the total cells and TUNEL-positive cells numbers.

Statistical analysis
Data were shown as mean ± SD. Student's t test or oneway ANOVA was used to compare the groups. P < 0.05 was considered significance.

MiR-542-3p was downregulated during cerebral infarction
We first established mouse model of MCAO at 12 h, 1 days, 3 days, 5 days, and 7 days. Then, FJC staining was used to determine degenerating neurons. FJC staining data showed that the number of degenerating neurons was increased at 12 h and peaked at 3 days after MCAO operation (Fig. 1a), which suggested that we successfully established a cerebral infarction model. Then, we performed microarray analysis, and the data showed the differentially expressed miRNAs in sham and MCAO operation of brain tissues, which showed a decrease of miR-542-3p in infarcted brain tissues (Fig. 1b). Following qRT-PCR analysis indicated that miR-542-3p was decreased after MCAO operation (Fig. 1c). Moreover, microarray analysis showed a significantly increase of TLR4 in MCAO mice comparing with sham mice (Fig. 1d). Then, qRT-PCR also suggested that TLR4 was induced in brain tissues after MCAO surgery (Fig. 1e). Meanwhile, bioinformatics analysis of the 3′ UTR of TLR4 predicted a conserved binding site for miR-542-3p. To verify that miR-542-3p interacts with the TLR4 mRNA, luciferase activity was performed in HEK293T cells. Cells transfected with miR-542-3p mimic decreased the luciferase activity in WT-TLR4 not in mutant type, which indicated the relationship miR-542-3p with TLR4 (Fig. 1f). And endogenous TLR4 was enriched in biotinylated miR-542-3p transfected HA1800 cells, which reveals a direct binding of TLR4 with miR-542-3p (Fig. 1g). The abnormal expression of miR-542-3p and TLR4 may be novel target for cerebral infarction.

MiR-542-3p was packaged into MSC exosomes
MiRNA in exosomes has attracted much attention due to the protection role of exosomes, and MSCs are a main source of exosomes [34]. In present study, MSCs were transfected with miR-542-3p mimic or miR-NC, and the expression level of miR-542-3p was detected by qRT-PCR (Fig. 4a). Exosomes isolated from supernatant of MSCs were identified by TEM (Fig. 4b), and particle size analysis (Fig. 4c). As well, exosome markers (CD63 and CD9) were detected using western blot (Fig. 4d). Then, the expression of miR-542-3p in isolated exosomes was assessed, which Fig. 1 The expression of miR-542-3p and TLR4 in brain tissues during cerebral infarction. Mice suffered from sham or MCAO operation with different time points (12 h, 1 days, 3 days, 5 days, and 7 days). a FJB staining was used to detect the degenerating neurons in brain tissues. b MicroRNA expression profiles in sham and MCAO (3 days) mice. Scale bar 20 μm. c qRT-PCR analyzed the expression of miR-542-3p in brain tissues of different groups. d MiRNA expression profiles in sham and MCAO (3d) mice. e TLR4 level in brain tissues of different groups was examined. f The pairing bases between TLR4 and miR-542-3p, and luciferase assay was used to determine the binding of TLR4 and miR-542-3p in HEK293 cells. g RIP assay for the binding of TLR4 and miR-542-3p in HA1800 cells. Data are mean ± SD; *P < 0.05, **P < 0.01. All experiments were repeated in triplicate showed an increase of miR-542-3p after miR-542-3p transfection of MSCs (Fig. 4e). In summary, miR-542-3p can be secreted by MSCs via exosomes.

Exo-miR-542-3p inhibited inflammation and injury via regulating TLR4 in glial cells
We then treated HA1800 cells with exo-miR-542-3p and OGD and transfected with TLR4 plasmid at the same time. Exo-miR-542-3p treatment increased miR-542-3p expression and inhibited TLR4 level, while TLR4 transfection reversed exo-miR-542-3p effects (Fig. 6a). Functional analysis has been suggested that exo-miR-542-3p increased cell viability and inhibited cell apoptosis, ROS production, and inflammation ( Fig. 6b-f). But, TLR4 transfection removed the benefit role of exo-miR-542-3p on HA1800 cells during OGD exposure (Fig. 6b-f). Taken together, exo-miR-542-3p from MSCs inhibited ischemic injury of brain via modulating TLR4. Fig. 2 AAV9-miR-542-3p injection attenuated MCAO induced injury. AAV9-miR-542-3p and AAV9-miR-NC were injected into paracele of mice at 1 h after MCAO or sham operation. Brain tissues were collected after 3 days of operation. a The expression of miR-542-3p in brain tissues after AAV9 injection was tested using qRT-PCR. b TTC staining indicated the infarcted area of brain tissues. c The brain water content in brain was detected. d Neurological grading scores of mice were evaluated. e FJB staining was used to detect the degenerating neurons. Scale bar 20 μm. f H&E staining showed the structure of brain in different groups. Scale bar 20 μm. g IHC staining for VEGFA expression in brain tissues. Scale bar 50 μm. h Inflammatory factors TNFα and MCP-1 were detected using qRT-PCR. i Western blot was used to test the protein expression of TLR4, TNFα, and IL 6. Data are mean ± SD; *P < 0.05, **P < 0.01. All experiments were repeated in triplicate

Discussion
Cerebral infarction is a series of cascade injury reactions caused by the damage of neurovascular components upon ischemia and hypoxia [35]. Cerebral infarction is mainly treated with tissue-type plasminogen activator thrombolytic therapy, but few patients actually recover their neurological function clinically. Exosomes are 40-100 nm in diameter, vesicles encapsulated by lipid bilayer membranes released from cells into the extracellular space [36]. Exosomes and miRNAs can effectively respond to the pathophysiological state of cells, and they have great potential in the diagnosis and treatment of tumors and cardiovascular and other systemic diseases [37]. Present study showed a downregulation of miR-542-3p and upregulation of TLR4 in infarcted brain tissues, and miR-542-3p interacted with TLR4 in HA1800 cells. Functionally, miR-542-3p protected against ischemic injury of brain tissue and nutritional deprivation damage of HA1800 cells. Interestingly, miR-542-3p can be packaged and secreted by MSCs. And exo-miR-542-3p from MSCs also showed a protective effect for MCAO-treated brain tissues or OGD-exposed HA1800 cells via modulating TLR4 level.
MiRNAs is a highly conserved non-coding RNA in eukaryotes [38]. By complementary pairing with the 3′ UTR of the target mRNA molecule, miRNA regulates the expression of target genes at the post-transcriptional level, leading to the degradation or inhibition of the translation of the target mRNA molecule [39]. Through this regulatory relationship, miRNA affects many aspects of life activities, such as early embryonic differentiation, development, cell proliferation, differentiation, apoptosis, Fig. 3 MiR-542-3p alleviated OGD induced glial cell damage. MiR-542-3p or miR-NC was transfected into HA1800 cells; then, cells were treated with or without OGD for 2 h. a MiR-542-3p expression in HA1800 cells was detected using qRT-PCR after transfection or OGD treatment. b Cell viability was tested by MTT. c TUNEL assay was used to calculate cell apoptotic level. Scale bar 20 μm. d ROS assay was used to determine the production of ROS in HA1800 cells. Scale bar 25 μm. e Inflammatory factors TNFα and MCP-1 were detected using qRT-PCR. f Western blot was used to test the protein expression of TLR4, TNFα, and IL 6. Data are mean ± SD; *P < 0.05, **P < 0.01. All experiments were repeated in triplicate inflammation, and so on [40]. MiRNA also plays multiple roles in various aspects of cerebral ischemia, regulating apoptosis, differentiation, vascular inflammatory response, ischemia-reperfusion injury, and so on. Ouyang et al. found that the target protein of miR-181 is a classic marker of GRP78, endoplasmic reticulum stress. The expression of miR-181 decreases and GRP78 increases during cerebral ischemia, which mediates apoptosis and promotes the development of brain injury [41]. Other studies have confirmed that miR-181a inhibitor treatment can significantly reduce the area of cerebral infarction, reduce the loss of neurological function, reduce the activation of NF-κB, reduce leukocyte infiltration, and protect against cerebral ischemic injury for a long time [42]. To explore the mechanism of ischemic injury of the brain, we constructed the mouse MCAO model, which is a commonly used animal model to detect cerebral ischemic diseases [43]. And FJC staining, TTC staining, and neurological grading scores were used to confirm the success of the MCAO model. To identify the regulating miRNA in cerebral infarction, we performed microarray and found a significant decrease of miR-542-3p in ischemic tissues. And the low expression of miR-542-3p in ischemic brain tissue has been show in the previous study [44]. Interestingly, TLR4, which potentially binds with miR-542-3p, was increased in infarcted brain tissue. And we performed luciferase assay and RIP assay to confirm the relation between miR-542-3p and TLR4. Our data indicated an interaction of miR-542-3p and TLR4 in glial cells. TLR4 is a key receptor of innate immunity, which mediates inflammatory response in multiple organs [45].
Then, we aimed to determine the role of miR-542-3p in cerebral infarction. AAV9-miR-542-3p was constructed to force miR-542-3p level in brain tissues. Adeno-associated virus vector is a safe, durable, and efficient gene manipulation tool, which has been widely used in the field of neurobiology [46]. The AAV9 subtype is a common type of brain tissue infection [47]. And in the present study, miR-542-3p level in brain tissues was prominently increased after AAV9-miR-542-3p injection. Then, we evaluated the function of miR-542-3p in infarcted brain, and our data showed that AAV9-miR-542-3p injection decreased infarcted size, reduced the number of degenerating neurons, and inhibited brain water content and neurological grading scores. Considering the key role of inflammation in cerebral infarction [48], we detected inflammatory response. Our results suggested that MCAO operation promoted the expression of inflammatory factors, and infiltration of inflammatory cells, while AAV9-miR-542- Fig. 4 MiR-542-3p can be packaged into MSC exosomes. a MiR-542-3p was transfected into MSCs, and the expression of miR-542-3p was determined. Exosomes in supernatant were isolated, and b TEM was used to show the morphology of exosomes. c Particle size analysis showed the size of exosomes. d Exosome markers (CD63 and CD9) and MSC marker (GM130) were detected using western blot. e The expression of miR-542-3p in isolated exosomes was determined. Data are mean ± SD; **P < 0.01. All experiments were repeated in triplicate 3p injection suppressed MCAO-induced inflammation in brain tissues. Furthermore, human glial cell line HA1800 cells were treated with OGD to mimic brain ischemia, and HA1800 cells were transfected with miR-542-3p mimics. In consistent with the results of in vivo experiments, miR-542-3p mimics inhibited OGD-induced cell apoptosis, ROS production, and inflammation.
Exosomes are vesicles secreted by cells with phospholipid bilayers. Different from other extracellular vesicles with a diameter of 100~1000 nm, exosomes originate Exosomal miR-542-3p remitted MCAO induced inflammation. MiR-542-3p was transfected into MSCs; exosomes in supernatant were isolated and injected into paracele of mice with or without MCAO. Brain tissues were collected after 3 days of operation. a MiR-542-3p expression in brain tissues was tested using qRT-PCR. b TTC staining for infarcted area of brain tissues. c The brain water content in brain was detected. d Neurological grading scores of mice were evaluated. e FJB staining was used to detect the degenerating neurons. Scale bar 20 μm. f H&E staining showed the structure of brain in different groups. Scale bar 20 μm. g IHC staining was used to detect VEGFA level brain in different groups. Scale bar 50 μm. h Inflammatory factors TNFα and MCP-1 were detected using qRT-PCR. i Western blot tested the protein expression of TLR4, TNFα, and IL 6. Data are mean ± SD; *P < 0.05, **P < 0.01. All experiments were repeated in triplicate directly from the cell membrane. Exosomes originate from the endosomal system of cells [45]. Exosomes mainly carry endogenous proteins, lipids, nucleic acids, and other substances, which are transmitted between cells through target cell internalization, receptor-ligand interaction, or lipid membrane fusion [49]. In 2007, Valadi et al. confirmed for the first time the existence of RNA, in exosomes, including mRNA, miRNA, and some non-coding RNA [50]. Exosomes can also carry proteins and peptides. Through proteome analysis, Laura Otero-Ortega's team has identified 2416 proteins in exosomes derived from MSCs, which are fully involved in molecular regulation, binding, and catalytic functions [51]. In addition, exosomes contain proteins involved in brain repair, including synaptic transmission, neural differentiation of neural stem cells, angiogenesis, neural projection, synaptic growth and so on. Studies have shown that after MSCs treatment, miR-133b levels are increased, and MSCs transmit miR-133b through exosomes to regulate nerve growth. MiR-133b overexpressed in exosomes secreted by MSCs can promote the recovery of nerve function and increase nerve plasticity by stimulating secondary secretion of exosomes from astrocytes [52]. MiR-17-92 derived from exosomes of MSCs promotes nerve recovery and neuroplasticity by activating PI3K/Akt/ mTOR/GSK-3β signaling pathway [53]. These evidences indicate the important role of MSC exosome-derived miR-NAs in cerebral ischemia, and we wondered whether miR-542-3p can be enveloped in MSCs and then secreted via exosomes. Exosomes were isolated from MSCs transfected with miR-542-3p, and we found a high expression of miR-542-3p in exosomes of MSCs supernatant, which revealed that miR-542-3p was packaged into exosomes and secreted into supernatant by MSCs. Moreover, exo-miR-542-3p exerted a protective role in MCAO-induced injury and OGD-treated HA1800 cells via modulating TLR4.
It has been proved that MSCs from different tissue sources can enhance the recovery of neurological function in animal models of stroke. Despite these findings, the difficulty of treatment in patients with acute stroke, Fig. 6 MiR-542-3p alleviated OGD induced glial cell damage. HA1800 cells were incubated with exo-miR-NC/exo-miR-542-3p and treated with or without OGD for 2 h and transfected with TLR4 plasmid at the same time. a MiR-542-3p expression in HA1800 cells was detected using qRT-PCR. b MTT assay for cell viability. c TUNEL assay for cell apoptotic level. Scale bar 20 μm. d ROS assay for the production of ROS in HA1800 cells. Scale bar 25 μm. e Inflammatory factors TNFα and MCP-1 were detected using qRT-PCR. f Western blot for the protein expression of TLR4, TNFα, and IL 6. Data are mean ± SD; *P < 0.05, **P < 0.01. All experiments were repeated in triplicate the risk of MSCs transplantation, and the high cost of maintaining stem cell activity limit the use of MSCs. In recent years, exosome has attracted wide attention because of its unique characteristics in the diagnosis and treatment of stroke. Because there is not enough clinical data to support the clinical application of exosome therapy, the diagnosis and treatment of exosomes is still in the theoretical and experimental stage.

Conclusion
In summary, MSC-derived exosome miR-542-3p prevented ischemia-induced glial cell inflammatory response via inhibiting TLR4. These results suggest possible therapeutic strategies for using exosome delivery of miR-542-3p to cure cerebral ischemic injury.