Experimental design
Thirty rats were randomly divided into 6 groups: healthy control group (n = 5), sham operation group (n = 5), MCAO group (n = 5), HF-MSCs group (n = 5), IGF-1 group (n = 5), and HF-MSCs + IGF-1 group (n = 5). Animals in the healthy control group were not treated, and those in the sham operation and MCAO groups were treated with 1 ml of saline. After the MCAO model was established (Fig. 1), the rats in the HF-MSCs transplantation group were injected with 1 ml of HF-MSCs (1×10⁷) through the tail vein, and those in the IGF-1 transplantation group were injected with 1 ml of IGF-1 (R&D Systems, MI, USA, 50 µg/ml) via the tail vein. Both HF-MSCs (1×10⁷) and IGF-1 (50 µg/ml) were injected into the tail veins of rats in the HF-MSCs + IGF-1 group.
On the 1st, 7th, 14th, 21st and 28th days after model establishment, the rats were assigned modified neurological severity scores (mNSS) and subjected to the beam-walking test. The samples were assayed for immunofluorescence on the 21st day, and immunohistochemistry analysis, western blot analysis and rt-PCR assays were performed on the 28th day, followed by statistical analysis (Fig. 1).
Isolation, identification, and in vitro fluorescence labeling of HF-MSCs
The skin of the whiskers of 1-week-old male Sprague-Dawley (SD) rats was excised after being disinfected with alcohol and digested with 3 mg/ml collagenase type I. The hair follicle bulge area was then separated under an anatomical microscope (Olympus, Japan), and rat HF-MSCs were cultured by tissue block attachment in F12 DMEM (HyClone) containing 10% foetal bovine serum (FBS, ScienCell Research Laboratories). Upon reaching 80% confluence, the cells were digested, subcultured at 37℃ and 5% CO2 and purified based on their adhesion to type I collagen, and cell density was adjusted to 1×10⁷ cells/ml. The expression levels of stem cell surface markers, including FITC-CD29, PE-CD90, PerCP-CD45 and APC-CD31, were measured by a flow cytometry (BD Biosciences) [11, 12, 27], and HF-MSCs were labeled with the green fluorescent dye PKH67 (SigmaAldrich, St. Louis, MO, USA). The labeled cells were observed under a fluorescence microscope (Olympus, Japan). Cell isotype control experiments were performed simultaneously to identify false positives.
Induced differentiation of HF-MSCs
To test the multidirectional differentiation function of the HF-MSCs, passage 3 cells were seeded in a 6-well culture plate. A lipogenic differentiation medium kit (Cyagen, RASMX-90031) and an osteogenic differentiation medium kit (Cyagen, RASMX-90021) for SD rat bone marrow mesenchymal stem cells (BMSCs) were used to induce osteogenesis and adipogenesis, respectively. After induction for 2 weeks to 1 month, the HF-MSCs were stained with Alizarin Red (Sigma-Aldrich) and Oil Red O (Sigma-Aldrich), and images were acquired by microscope (Olympus, Japan). Currently, there are no osteogenic or adipogenic media for inducing differentiation of HF-MSCs commercially available. As HF-MSCs and BMSCs are both mesenchymal stem cells, BMSC differentiation media were needed in this study.
Middle cerebral artery occlusion model
Adult male clean-grade SD rats weighing 240–260 g were obtained from the Animal Experiment Center of the Second Affiliated Hospital of Harbin Medical University. All relevant procedures were approved by the Ethics Committee of Harbin Medical University and were in compliance with National Institutes of Health (NIH) guidelines for the use and care of laboratory animals. After 24 hours of fasting and 2 hours of water deprivation, the rats were anesthetized with ketamine (60 mg/kg) and xylazine (10 mg/kg) and placed in the supine position for surgery, with their limbs and heads being fixed. The proximal common carotid artery (CCA) and the external carotid artery (ECA) of each rat were ligated, and the internal carotid artery (ICA) was clamped with arteriolar forceps. A silicon-coated monofilament was slowly inserted into the ICA through the CCA. After loosening the arterial forceps, the monofilament finally reached the middle cerebral artery [28], and insertion of the monofilament was halted as soon as resistance was felt. During the surgery, the body temperature of the rat was maintained at 37 ± 1℃ with an electric blanket. After one hour, part of the monofilament was removed, but it was retained in the ICA. Finally, the incision was sutured, and the exposed portion of the monofilament was cut to prevent the rat from removing it on its own. Rats in the sham-operated group underwent only vascular dissection. The occurrence of hemiplegia was indicative of successful model establishment.
2,3,5-triphenyltetrazolium chloride (TTC) staining
Ischemic brain injury was assessed by TTC staining at 24 hours after model establishment. The brain tissue was quickly removed from ice, flash frozen at -80℃ for 1 minute, sliced into an average of five consecutive coronal sections and incubated in 2% TTC (St Louis, Missouri). A digital camera was used to image the brain tissue sections.
Hematoxylin-eosin (HE) staining
HE staining can be used to distinguish normal cells from necrotic cells. Brain tissues were paraffinized and then cut into successive coronal sections. According to a standard protocol, the specimens were stained with HE, and observed under a light microscope.
Beam-walking test and modified neurological severity scores (mNSS)
Motor coordination in rats can be evaluated by the beam-walking test and mNSS. For the beam-walking test, rats were placed on a high, narrow beam and allowed to walk, and hemiplegia and even falling were observed. The mNSS test was used to examine the behaviors of the model rats, which received scores ranging between 0 and 18 [29, 30]. Rats with mild or severe symptoms and even those that died were excluded from the analysis. On days 1, 7, 14, 21 and 28 after the operation, the beam-walking test and mNSS test were performed to evaluate the improvement of motor function.
Immunofluorescence
After heart perfusion, the brain tissue was fixed in 4% paraformaldehyde (pH 7.4) at 4℃ for 24 hours and then dehydrated in 20% and 30% sucrose solutions until it sank to the bottom of the container. The brain tissue was sectioned into coronal frozen sections (8 µm) and immediately immobilized in acetone for 10 minutes. After overnight incubation at 4℃ with anti-neuronal nuclei protein (anti-NeuN) (Abcam, ab177487, 1:100), anti-neurofilament heavy polypeptide (anti-NF-H) (Abcam, ab207176, 1:200), anti-doublecortin (anti-DCX) (Abcam, ab207175, 1:100) and anti-glial fibrillary acidic protein (anti-GFAP) (Abcam, ab33922, 1:100) antibodies, frozen brain tissue sections were incubated with normal goat serum for 20 minutes at room temperature, washed with PBS 3 times, and incubated with rhodamine-conjugated goat anti-mouse IgG (ZSGB, ZF-0313) and anti-rabbit IgG (ZSGB, ZF-0316) for 1.5 hours. The brain sections were counterstained with the DAPI (Sigma-Aldrich) at 37℃ for 10 minutes. The HF-MSCs were observed under a laser scanning confocal microscope, and the positive cells’ average number was calculated from counts of five randomly selected regions.
Immunohistochemistry
After heart perfusion, the brain tissue was embedded in paraffin and sectioned (10 µm) before being dewaxed and hydrated in xylene and ethanol solutions. The sections were heated at 95℃ for 10 minutes, incubated overnight with anti-NeuN (1:400), anti-NF-H (1:200), anti-DCX (1:50), and anti-GFAP (1:400) antibodies at 4℃, and then reacted with the biotinylated secondary antibodies for 20 minutes at room temperature. After the sections were incubated in 0.05% diaminobenzidine (DAB) and PBS, the product of this reaction turned brown, at which point the sections were counterstained with hematoxylin. After dehydration in an ethanol solution, the sections were sealed on glass slides and analyzed under an optical microscope, and semiquantitative analysis was conducted with ImageJ (National Institutes of Health, Bethesda, USA).
Western blot
Fresh brain tissue was taken, and the corresponding volume of lysate was added according to the quality and volume of each sample and then centrifuged at 12000 rpm and 4℃ for 10 minutes. The protein concentration was determined using a bicinchoninic acid (BCA) protein assay kit. The electrophoretic device was assembled, and the electrophoretic protein sample (every 40 µg) was transferred to a polyvinylidene fluoride (PVDF) membrane, which was incubated with TBST buffer and 5% m/V skimmed milk powder under gentle shaking and then incubated overnight at 4℃ with anti-NeuN (1:500), anti-NF-H (1:1000), anti-DCX (1:500) and anti-GFAP (1:500) antibodies overnight. After the membrane was incubated with the secondary antibody (1:5000) coupled to horseradish peroxidase at 37℃ for 45 minutes, the substrates were detected with an ECL chemiluminescence detection kit. The film was scanned, and the optical density of the target band was analyzed.
Real-time polymerase chain reaction (rt-PCR)
Total RNA was harvested from frozen brain tissue with the TRIzol (Invitrogen, Carlsbad, CA, USA) extraction kit in accordance following the manufacturer’s instructions, and the RNA concentrations in each sample were determined using a NANO 2000 photometer. The RNA samples were reverse-transcribed by PCR to obtain the corresponding cDNA, and β-actin served as an internal reference. Fluorescence quantitative analysis was performed on an ExicyclerTM 96 fluorescence quantizer made by Bioneer (Korea) under the following conditions: pre-denaturation at 94 for 5 minutes, followed by 40 cycles of 94℃ for 10 seconds, and an annealing step at 60℃ for 20 seconds. The gene expression levels of neural-specific markers in each group were compared using the 2− ΔΔ CT method. All PCRs were repeated three times, and the existence of a single peak was verified by the melting curve. The primers are shown in Table I.
Table I. rt-PCR primer sequences
Gene of interest | Direction | Primer sequence (5’-3’) |
NeuN | Forward | GGTGCTGAGATTTATGGAGG |
NeuN | Reverse | CGATGGTATGATGGTAGGGA |
GFAP | Forward | TGACTATCGCCGCCAACTGC |
GFAP | Reverse | CTGGTAACTCGCCGACTCCC |
NF-H | Forward | AGTGAACACGGATGCTATGC |
NF-H | Reverse | CAGTGACTCCTTGGTGCTTT |
DCX | Forward | AGAGGGTCACGGATGAATGG |
DCX | Reverse | GAGGCAGGTTGATGTTGTCG |
β-actin | Forward | GGAGATTACTGCCCTGGCTCCTAGC |
β-actin | Reverse | GGCCGGACTCATCGTACTCCTGCTT |
Statistical analysis
All data are expressed as the mean ± standard error of the mean (SEM) from three repeated experiments and were analyzed by Student’s t-test, Bonferroni’s multiple comparison test, a two-factor factorial design and ANOVA in a fully randomized manner. All statistical analyses were performed with SPSS 24.0 (SPSS Inc, Chicago, IL, USA) and Prism 8.0 (GraphPad), and differences between the means were considered statistically significant for P < 0.05.