Peroxiredoxin 2 activates microglia by interacting with Toll-like receptor 4 after subarachnoid hemorrhage

All animal procedures were approved by the Ethics Review Committee for Animal Experimentation at Drum Tower Hospital (Nanjing, China) and performed in accordance with the institutional guidelines. Neonatal C57BL/6JNju mice were sacrificed for primary cortical neurons and microglia. For primary TLR4-KO microglia, neonatal C57BL/10ScNJNju mice were used. Two strains of mice were purchased from Nanjing Biomedical Research Institute of Nanjing University.

Primary cortical neural cells were cultured as described previously [10]. In brief, cerebral cortex was isolated from brains of neonatal (1–3 days) mice. The leptomeninges and white matter were removed, and brain tissue were digested with 0.125% trypsin (Gibco) for 5 min at 37 °C. Subsequently, the suspension was filtered through a 40-μm strainer (Millipore) and centrifuged at 1500 r/min for 10 min. The remaining cells were resuspended in Dulbecco’s modified Eagle’s medium (DMEM) with 10% fetal bovine serum (FBS) and penicillin-streptomycin. For neurons, cell suspensions were seeded into poly-d-lysine-coated six-well plates, and their culture medium was replaced after 2 h with Neurobasal Medium containing 0.5 mmol/L GlutaMAX-I and 2% B27 supplement (Gibco). Neuronal cultures were used on day 8 in vitro (DIV8). Primary microglia were obtained as described [11]; cells were seeded in flasks coated with poly-d-lysine to obtain mixed glial cultures. When the glial cultures reached confluency for 3 days, the flasks were shaken 2 h at 250 rpm. The floating cells were collected and seeded in six-well plates to obtain microglia. Microglial cultures were used on DIV14. For neurons and microglia co-culture system, microglia were seeded in Transwell (Corning, pore size = 0.4 μm) upper chamber and the neurons were seeded in the plates. Co-culture medium was DMEM with 10% FBS. The serum was reduced to 2% before any treatment. The co-culture system was harvested 24 h after indicated intervention. The purity of primary neuron and microglia was more than 90% (Additional file 1: Figure S1).

Mouse hemoglobin (Sigma) was used to produce oxyhemoglobin as per the manufacturer’s instruction. In brief, the reduced hemoglobin was prepared by gel filtrating 1 mmol/L hemoglobin solution with sodium dithionite in a column containing Sephadex G-25 (Sigma). Then, the reduced hemoglobin was saturated with oxygen gas. Sodium dithionite was then removed by dialysis against 100 volumes of oxygen-saturated phosphate buffer. Oxyhemoglobin were achieved and stored at − 80 °C.

To mimic SAH in vitro, the co-culture system was exposed to oxyhemoglobin (OxyHb) at a concentration of 25 μmol/L. The treatment groups were prepared with OxyHb-exposed neuron-microglia co-culture systems respectively adding 1, 5, and 10 μg/mL recombinant Prx2 (Abcam) for 24 h. MyD88 inhibitor ST-2825 (MCE) was premixed with culture medium at a concentration of 10 and 100 μmol/L, and the NF-κB inhibitor pyrrolidine dithiocarbamate (PDTC) was premixed with microglial culture medium at a concentration of 80 μmol/L 24 h before the in vitro SAH model was induced respectively.

The Cytotoxicity Detection Kit (LDH) (Roche) was used to measure neural cell viability as per the manufacturer’s instruction. After treatment, 100 μL of culture medium was transferred to a 96-well plate; the reaction mixture was added to each well and incubated for 30 min at room temperature. The absorbance was measured at a wavelength of 490 nm, and the reference wavelength was 690 nm.

RNA extracted using TRIzol Reagent (TAKARA) was reverse transcribed into cDNA with the Reverse Transcriptase Reagent (TAKARA). Quantitative real-time PCR analysis was performed with UltraSYBR Mixture using the LightCycler 96 Real-Time PCR System (Roche). The primers are as follows: IL-1β, 5′-GCCTGTGTTTTCCTCCTTGC-3′ (forward), 5-TGCTGCCTAATGTCCCCTTG-3′ (reverse); TNF-α, 5′-CGGGCAGGTCTACTTTGGAG-3′ (forward), 5′-ACCCTGAGCCATAATCCCCT-3′ (reverse); IL-6, 5′-GAGACTTCCATCCAGTTGCCT-3′ (forward), 5′-TGGGAGTGGTATCCTCTGTGA-3′ (reverse); and Rpl5, 5′-GGAAGCACATCATGGGTCAGA-3′ (forward), 5′-TACGCATCTTCATCTTCCTCCATT-3′ (reverse). Rpl5 was used as housekeeping gene. After 95 °C for 30 s, 40 PCR cycles were performed, each consisting of a denaturation step (95 °C, 5 s) and an annealing step (60 °C, 30 s).

The primary microglial culture medium of each group was collected, and the quantities of IL-1β, TNF-α, and IL-6 were determined using enzyme-linked immunosorbent assay kits (Boster) according to the manufacturer’s instructions.

Immunofluorescence staining was performed according to our previous study. Briefly, cultured cells on coverslips were fixed in 4% paraformaldehyde. Following treatment with 0.1% Triton X-100, the samples were blocked by 5% bovine serum albumin (BSA) prior to incubation with primary antibody overnight. The samples were washed three times with 0.5% phosphate-buffered saline with Tween-20 (PBST) and then were incubated with proper secondary antibodies. After three washes again, the coverslips were counterstained by 4,6-diamidino-2-phenylindole (DAPI) for 2 min. The following antibodies were used: anti-Iba1 (1:200, Abcam) and NeuN (1:200, Millipore). Pictures were acquired with a fluorescence microscope (Zeiss) under the same exposure time 0.5 s.

Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) staining was conducted by using a TUNEL detection kit according to the manufacturer’s instructions (Roche). Coverslips were incubated with primary antibody against NeuN (1:100, Millipore) at 4 °C overnight. After washed three times with PBST, the coverslips were incubated with TUNEL reaction mixture for 45 min prior to be counterstained by DAPI. The positive cells were identified, counted, and analyzed by two investigators blinded to the grouping.

Cells after indicated treatment were washed three times with PBS and lysed in radioimmunoprecipitation assay buffer (RIPA) containing protease and phosphatase inhibitor cocktails (Roche). Protein concentrations were determined with the BCA kit (Beyotime). Equal amounts of protein (10 μg) per lane were separated by SDS-polyacrylamide gel and transferred to a polyvinylidene difluoride (PVDF) membrane. The membrane was blocked in 5% skim milk for 2 h at room temperature and incubated overnight at 4 °C with primary antibodies against Iba-1 (1:1000, Abcam), MyD88 (1:2000, Abcam), p65 (1:1000, Cell Signaling Technology), Prx2 (1:5000, Abcam), and TLR4 (1:200, Santa Cruz) in 0.1% TBST containing 5% BSA. After the membrane was washed, it was incubated with HRP-conjugated secondary antibody for 2 h at room temperature. Detection was performed by Immobilon Western Chemiluminescent HRP Substrate (Millipore), according to the manufacturer’s instruction. Band intensities were quantified using the ImageJ software.

Eight control patients and 24 SAH patients from Drum Tower Hospital were included prospectively after written informed consent was obtained from all patients or their family members. This study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of Drum Tower Hospital. CSF samples were collected through lumbar puncture between 24 and 72 h after the SAH. Immediately after collection, samples were centrifuged at 3000 rpm for 10 min at 4 °C and the supernatants were collected. An equal amount (50 μL) of sample per lane was added and detected by Western blot.

All data were expressed as the mean ± SD. Statistical comparisons were performed using one-way ANOVA. Differences between experimental groups were determined by Student’s t test. A value of P < 0.05 was considered statistically significant.

Single-neuron culture and neuron-microglia co-culture system were treated with different doses of recombinant Prx2 respectively. The neuronal damage was aggravated after incubation with oxyhemoglobin (OxyHb) for 24 h, and the damage became more severe when it was co-cultured with microglia (Fig. 1a). In neuron-microglia co-culture system, exposure to Prx2 10 μg/mL alone could cause neuronal disintegration. As shown in Fig. 1b, treating single-neuron culture with Prx2 alone did not affect the neuronal cytotoxicity. Incubation with OxyHb produced obvious neuronal cytotoxicity; however, when compared with the OxyHb group, there were no significant differences in the OxyHb+Prx2 groups. In neuron-microglia co-culture system, a Prx2 dose-dependent neurotoxicity was observed. The OxyHb+Prx2 10 μg/mL group was the most severe one. Interestingly, treating the co-culture system with Prx2 alone could also cause neurotoxicity (Fig. 1c). To determine the microglial activation, we assessed the Iba-1 expression in primary microglia by Western blot. As shown in Fig. 1d, Prx2 significantly increased the expression of Iba-1 after in vitro SAH.

To investigate the mechanism of microglia-dependent Prx2-induced neurotoxicity, we detect the interleukin 1β (IL-1β), interleukin 6 (IL-6), and tumor necrosis factor-α (TNF-α) mRNA expression in microglia and their concentrations in the co-culture medium. As shown in Fig. 2a, the IL-1β, IL-6, and TNF-α mRNA level in cultured microglia were elevated after OxyHb incubation for 24 h. Moreover, incubation of OxyHb along with 10 μg/mL Prx2 promoted the mRNA expression of IL-1β and IL-6. However, TNF-α mRNA level remained unchanged in the OxyHb+Prx2 group compared with the OxyHb group. Consistent with the results above, the concentration of IL-1β and IL-6 in the microglial culture medium were significantly increased after SAH. Prx2 treatment markedly increased their concentration in culture medium when compared with the OxyHb group (Fig. 2b). Phase-contrast microscope images revealed the morphological remodeling of microglia in response to OxyHb and OxyHb+Prx2 treatment. In the control group, microglia have highly ramified morphology with thin processes. Upon OxyHb stimuli, the processes of microglia retracted and thickened, which indicated the microglial activation and acquirement of the ability of secreting pro-inflammatory cytokines. Compared with the OxyHb group, incubation of OxyHb+Prx2 further activated the microglia (Fig. 2c).

The TLR4/MyD88/NF-κB signaling pathway was involved in the activation of microglia. Since peroxiredoxin 1 (Prx1) could interact with TLR4, we assumed that Prx2 may activate microglia after SAH by interacting with TLR4. We detected the expression of TLR4, MyD88, and p65 in cultured microglia by immunofluorescence and Western blot. After SAH, the expression of TLR4, MyD88, and p65 were elevated (Fig. 3a). The Western blot results showed that Prx2 increased the expression of TLR4, MyD88, and p65 when compared with the OxyHb group (Fig. 3b). To confirm the effects, we co-cultured neuron with TLR4-KO microglia and ST-2825-treated microglia. We found that TLR4 KO eliminated the Prx2-induced neuronal cytotoxicity (Fig. 4a). Inhibition of MyD88 by ST-2825 at the concentration of 100 μmol/L significantly remitted the neuronal cytotoxicity caused by Prx2 (Fig. 4b). PDTC was used to inhibit NF-κB activity in microglia. After treated by PDTC at the concentration of 80 μmol/L, the Prx2-induced microglia-mediated neuronal cytotoxicity was eliminated (Fig. 4c).

TUNEL staining was performed to demonstrate the role of Prx2-induced microglia-mediated neuronal injury (Fig. 5a). In neuron-microglia co-culture system, the fraction of apoptotic neurons was significantly increased after in vitro SAH model was induced. Compared with the OxyHb group, the number of apoptotic neurons was increased in the OxyHb+Prx2 group. We then co-cultured neurons with TLR4-KO microglia; the Prx2-induced microglia-mediated neuronal apoptosis was attenuated (Fig. 5b).

To investigate the relationship between Prx2 in CSF with the severity of brain injury, we detected the Prx2 level in aneurysmal SAH patients’ CSF by Western blot. The CSF collected from patients who underwent hip arthroplasty before surgery were used as control. We found that the Prx2 level in CSF after SAH was elevated significantly compared with the control group. There were no obvious differences between Hunt-Hess grade I–II group and grade III group. However, in grade IV–V group, the Prx2 level in CSF was increased significantly compared with grade III group (Fig. 6). These results indicated that the level of Prx2 in CSF of aneurysmal SAH patients was positively correlated with the severity of brain injury, and it may also correlate with prognosis of SAH patients.