Neural stem cell conditioned medium alleviates Aβ_25-35 damage to SH-SY5Y cells through the PCMT1/MST1 pathway.

Alzheimer's disease (AD) is a progressive, neurodegenerative disease. Accumulating evidence suggests that protein isoaspartate methyltransferase 1 (PCMT1) is highly expressed in brain tissue (substantia nigra, blue plaque, paraventricular nucleus). In this study, we investigated the effect of neural stem cell conditioned medium alleviates Aβ25-35 damage to SH-SY5Y cells by PCMT1/MST1 pathway. Results demonstrated that Aβ25-35 significantly decreased the cell viability in time and dose dependent manner. However, Neural stem cell-complete medium (NSC-CPM) or NSC-CDM had inhibitory effect on toxicity when fibrillation of Aβ25-35 occurred in their presence and NSC-CDM had a better inhibitor result. An increase of the PCMT1 expression levels was found in Aβ25-35 + NSC-CDM group. sh-PCMT1 significantly reduced the PCMT1, the cell viability and inhibited the protective effect; induced apoptosis and increased the expression of p-MST1. Overexpression of PCMT1 group reversed the effect of Aβ25-35 inhibited the cell viability and Aβ25-35 induced the apoptosis. In conclusion, NSC-CDM corrects the damage of Aβ25-35 to cells by increasing PCMT1, reducing MST phosphorylation.

Introduction

Alzheimer disease (AD) is a progressive neurodegenerative disease that is characterized by several pathological features, including the formation of senile plaques (SPs) between nerve cells and neurofibrillary tangles within neurons (NFTs). NFTs consist of abnormal aggregation of amyloid β protein (Aβ), which is mainly composed of the microtubule-associated protein tau.[1,2] AD is a multifactorial disease with multiple risk factors. The pathological mechanisms of AD include the deposition of amyloid Aβ,[3] astroglial degeneration, hyperphosphorylation and accumulation of the tau protein,[4] neuronal dystrophy, oxidative stress,[5] mitochondrial dysfunction,[6] biological metal homeostasis, and decreased levels of acetylcholine (ACh). However, the exact molecular mechanisms underlying AD are still unknown and no cure has yet been identified for the disease.

Protein L-Isoaspartate (D-Aspartate) O-Methyltransferase (PCMT1) is highly expressed in brain tissue, particularly in the substantia nigra, blue plaque, and paraventricular nucleus.[7] Increasing evidence suggests that PCMT1 plays a key role in the broad-spectrum regulation of many physiological processes, including longevity, apoptosis, heat shock response, and different types of oxidative stress.[8,9] In recent years, studies have confirmed that short-term tissue damage in the brain, such as traumatic brain injury (TBI) or subarachnoid hemorrhage (SAH), leads to up-regulation of PCMT1 activity within 72 h. PCMT1 upregulation is associated with anti-apoptosis effects on neurons and the reduction of levels of neurodegenerative factors, including brain edema.[10,11]

Using neural stem cell conditioned medium (NSC-CDM), which contains a variety of paracrine substances of neural stem cells, has become a new treatment strategy to replace neural stem cell transplantation. Studies have shown that NSC-CDM increases macrophage M2 expression in vitro, reduces activation of M1 macrophages, and inhibits the release of multiple inflammatory factors.[12] Similarly, in vivo experiments have shown that injection of NSC-CDM into rats with spinal cord injury increases bridging between the corticospinal tract and interneurons, reduces neuronal apoptosis, and promotes motor function recovery.[13]

NSCs are the precursor cells or the source of neuronal differentiation. NSCs have strong self-renewal abilities and significant paracrine effects during differentiation. Therefore, we used the conditioned medium containing NSC paracrine products as a method to replace cell transplantation. We aimed to investigate the protective role of NSC-CDM against Aβ25-35-induced cytotoxicity, including its effects on apoptosis, cell viability, oxidative stress, and damage to the mitochondrial ultrastructure. PCMT1 is conserved in mammals and may play an important role in the process of self-repair in NSCs. Therefore, we hypothesized that NCSCDM may regulate neuronal apoptosis through regulation of the PCMT1/MST1 pathway. These results provide a new target for AD therapy using a single intervention that has multiple effects.

Materials and Methods

Aβ25-35 preparation

Five mg of Aβ25-35 (Sigma-Aldrich, St. Louis, MO, USA) was dissolved in 5 mL double distilled water. The solution was sterilized by filtration (0.22 μm) under sterile conditions and placed in a 37°C incubator for 7 days. A small sample was taken for protein concentration determination and the solution was stored at -20°C for later use.

Cell culture and treatment

Human SH-SY5Y cells in the logarithmic growth phase were collected, counted, and resuspended in DMEM/F-12 complete medium (10% FBS and 1% penicillin and streptomycin) at a concentration of 1×105 cells/mL. The cells were seeded in 6-well plates with 2 mL of cell suspension per well. Cells were incubated at 37°C with 5% CO2 overnight. After the cells were fully attached, the medium was removed and processed as follows: control group, 2 mL of DMEM/F-12 medium containing 10% FBS was added to a 6-well plate; Aβ25-35 group, Aβ25-35 and DMEM/F-12 medium containing 10% FBS were added to a 6-well plate; Aβ25-35 + neural stem cell complete medium (NSC-CPM) group, Aβ25-35 and 10% FBS-containing neural stem cell complete medium were added to the 6-well plate; Aβ25-35 + neural stem cell conditioned medium (NSC-CDM) group, Aβ25-35 and 10% FBS-containing neural stem cell conditioned medium were added to a 6-well plate. The final concentration of Aβ25-35 was 40 μM for all Aβ25-35-containing groups. The method of isolating and culturing NSCs and NSCCDM were performed as previously described.[14]

CCK-8 analysis

SH-SY5Y cells were grown at 2-4×104 cells per well in 96- well microplates. The CCK-8 solution (Sigma-Aldrich) was then added to the medium to a final concentration of 0.5 mg/mL and incubated for 4 h at 37°C. Absorbance measurements were taken at 450 nm.

Apoptosis analysis

Using an in situ cell death detection kit (Roche, Mannheim, Germany), the cells were grown on coverslips and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) was performed. After TUNEL labeling, the cells were observed using a light microscope (Olympus, Tokyo, Japan) to detect apoptotic cells at 400x magnification, with a view size area of 0.344 mm[2].

PCMT1 sh-RNA and plasmid transfections

A PCMT1 overexpression vector and short hairpin RNA (shRNA) targeting PCMT1 were synthesized by GenePharma Co (Shanghai, China). Cells were transfected with the above vectors using Lipofectamine 3000 Reagent (Invitrogen, Carlsbad, CA, USA).

Group 1: A, normal control (control) group; B, transfection negative control (NC) group; C, transfection with sh-PCMT1 (sh- PCMT1); D, 10% FBS-containing neural stem cell conditioned medium cultured with SH-SY5Y cells after transfection of sh- PCMT1 (sh-PCMT1+NSC-CDM) group. Six h after transfection, the media in the wells was discarded and fresh medium was added (for groups A, B, and C DMEM/F-12 complete medium was added, and for group D 10% FBS-containing neural stem cell conditioned medium was added).

Group 2: A, transfection of GFP empty plasmid (vector); B, transfection of PCMT1 overexpression plasmid (PCMT1-OV); C, Aβ25-35 was added to a final concentration of 40 μM after transfection of a GFP empty plasmid (Aβ25-35); D, transfection of PCMT1 overexpression plasmid followed by addition of Aβ25-35 at a final concentration of 40 μM (Aβ25-35+PCMT1-OV). Six h after transfection, the medium in the wells was discarded and fresh DMEM/F-12 complete medium was added, and for groups C and D 40 μM Aβ25-35 was also added.

qRT-PCR

RT-PCR was used to detect mRNA expression of PCMT1. The total RNA was extracted by nano-magnetic beads using the MagBeads Total RNA Extraction Kit (Tiangen, Beijing, China) and converted to cDNA using reverse transcription. Reaction conditions were as follows: pre-denaturation at 95°C for 15 min, deformation at 95°C for 5 s, annealing at 60°C for 30 s, and 45 total cycles. The specificity of the primer was determined by the dissolution curve of the PCR products and the relative expression amount of mRNA = 2-ΔΔCt*100%.

Western blot analysis

Proteins were examined via western blotting using antibodies against PCMT1 (1:500, Abcam, Cambridge, MA, USA), Bax (1:2,000, Abcam), Bcl 2 (1:1,000, Abcam), cleaved caspase 3 (1:500, Abcam), caspase 3 (1:500, Abcam), and MST1 (1:1,000, CST). β-actin (1:5,000, Sigma-Aldrich) served as a loading control. A horseradish peroxidase (HRP)-labeled secondary antibody (1:1,000, Santa Cruz Biotechnologies, Santa Cruz, CA, USA) was used and cultured with the cells for 1 h at 25°C. Quantification of band density was performed using the LI-COR Odyssey infrared imaging system (LI-COR Biosciences, Lincoln, NE, USA).

Statistical analysis

The GraphPad 8.0 software was used to analyze the data. All experiments were repeated three times and the mean ± standard deviation (SD) were used in the analyses. ANOVA was used to determine whether differences existed among the experimental groups, and a P<0.05 considered statistically significant.

Results

Protective effect of different media on Aβ25-35-associated damage

SH-SY5Y cells were treated with Aβ25-35 (20 μM) for different lengths of time. Cell survival decreased in a time-dependent manner. At 36 h, cell viability decreased to 58.62±1.26 %, as compared with the control group (P<0.05) (Figure 1A). Different concentrations of Aβ25-35 treated SH-SY5Y cells for 24 h. The cell survival rate gradually decreased with increasing concentration of Aβ25-35. At 40 μM Aβ25-35 cell viability decreased to 56.62±1.26 %, as compared with that of the control group (P<0.05) (Figure 1B). As shown in Figure 1C, Aβ25-35 significantly decreased cell viability as compared to that of the control group (P<0.001). However, NSCCPM and NSC-CDM had inhibitory effects on the toxicity induced by Aβ25-35 treatment (P<0.01 and P<0.01, respectively). Aβ25-35 induced apoptosis in the SH-SY5Y cells, as indicated by the increased abundance of TUNEL-positive nuclei in these cells (Figure 1 D,E); condensed nuclei and nuclear fragmentation of apoptotic cells were also clearly observed. The number of TUNEL-positive nuclei was significantly lower in the NSC-CPM and NSC-CDM+Aβ25-35-treated cells.

Figure 1.

Protective effect of different media on Aβ25-35 damage. A) CCK-8 detection of Aβ25-35 (20 μM) treatment of SH-SY5Y cells for 12, 24, 36 and 48 h. B) CCK-8 detection of Aβ25-35 (0, 10, 20, 30, 40 and 50 μM) treatment of SH-SY5Y cells for 24 h. C) The cell viability of SH-SY5Y cells was evaluated by CCK-8 assays in control, Aβ25-35 (40 μM), Aβ25-35 (40 μM) + Neural stem cell-complete medium (NSC-CPM) and Aβ25-35 (40 μM) + Neural stem cell-conditioned medium (NSC-CDM) group for 24 h. D,E) Apoptotic rates of cells were labeled with TUNEL assay in control, Aβ25-35 (40 μM), Aβ25-35 (40 μM) +NSC-CPM and Aβ25-35 (40 μM) + NSC-CDM group for 24 h. Scale bars: 100 μm. *P<0.05, **P<0.01, ***P<0.001.

Effect of different mediums on PCMT1 expression

Western blotting showed a decrease in PCMT1 expression levels in the Aβ25-35 group as compared with the control group (P<0.01). These effects were ameliorated by NSC-CPM or NSCCDM (Figure 2 A,B). In addition, we found that expression of PCMT1 was significantly decreased in the Aβ25-35 group as compared to the control group (P<0.01), while the NSC-CPM and NSC-CDM+Aβ25-35 groups had increased PCMT1 levels (P<0.05) (Figure 2C).

Figure 2.

Effect of different mediums on PCMT1 expression. A,B) PCMT1 protein expressions were evaluated by Western blot in control, Aβ25-35 (40 μM), Aβ25-35 (40 μM) +NSC-CPM and Aβ25-35 (40 μM) + NSC-CDM group for 24 h. C) Expression of PCMT1 mRNA was evaluated by qRT-PCR in control, Aβ25-35 (40 μM), Aβ25-35 (40 μM) +NSC-CPM and Aβ25-35 (40 μM) + NSC-CDM group for 24 h. *P<0.05, **P<0.01, ***P<0.001.

Silencing and overexpression of PCMT1

As shown in Figure 3 A-C, the mRNA and protein expression of PCMT1 were decreased in the sh-PCMT1 group as compared to the NC group. In the sh-PCMT1-3 (72 h) group the mRNA and protein expression of PCMT1 were minimal. In addition, the mRNA and protein expression of PCMT1 were both markedly increased in the PCMT1-OV group as compared to the vector group. These results indicate that transfections were effective for silencing and overexpression of PCMT1.

Figure 3.

Detection of silent expression and overexpression of PCMT1. A) Expression of PCMT1 mRNA expression was evaluated by qRT-PCR in different time (24 h, 48 h, 72 h) of sh-PCMT1. B,C) Expression of PCMT1 protein expression was evaluated by Western blot in different time (24 h, 48 h, 72 h) of sh-PCMT1. D) Expression of PCMT1 mRNA expression was evaluated by qRT-PCR in overexpression PCMT1. E,F) Expression of PCMT1 protein expression was evaluated by Western blot in overexpression PCMT1 for 48 h. *P<0.05, **P<0.01, ***P<0.001.

Effect of PCMT1 on cell viability and apoptosis

CCK-8 analysis showed that after knockdown of PCMT1 using sh-PCMT1, the cell viability was significantly reduced and the protective effect was inhibited, as compared to the NC group (P<0.01 and P<0.05, respectively). Greater cell viability was observed in the PCMT1-OV group as compared with the vector group (P<0.05) and the effect of Aβ25-35 inhibition of cell viability was reversed in the PCMT1-OV group (P<0.01) (Figure 4A). In addition, the TUNEL assay showed that the numbers of apoptotic cells were markedly higher in the sh-PCMT1 group and sh- PCMT1+NSC-CDM group as compared to the NC group; however, overexpression of PCMT1 reversed the effect of Aβ25-35 - induced apoptosis (Figure 4 B,D). Western blotting results showed that sh-PCMT1 significantly reduced PCMT1 expression and the ratio of Bcl 2/Bax and increased the ratio of cleaved caspase 3/caspase 3 and p-MST1/T-MST1 in the sh-PCMT1 group and the sh- PCMT1+NSC-CDM group as compared to the NC group. Overexpression of PCMT1 reversed the effect of Aβ25-35 on protein expression of these factors (Figure 4E).

Figure 4.

PCMT1 promoted the cell viability and inhibited apoptosis of SH-SY5Y cells. A) Viability of SH-SY5Y cells was evaluated by CCK-8 assays in control, negative control (NC), sh-PCMT1 (PCMT1 shRNA, 48 h), sh-PCMT1 (48 h) +NCS-CDM, vector (expression of the GFP genome), PCMT1-OV (overexpression plasmid, 48 h), Aβ25-35 (40 μM) and PCMT1-OV (48 h) +Aβ25-35 (40 μM) group. BD) Apoptotic rates of cells were labeled with TUNEL assay in control, NC, sh-PCMT1 (48 h), sh-PCMT1 (48 h)+NCS-CDM, vector, PCMT1-OV (48 h), Aβ25-35 (40 μM) and PCMT1-OV (48 h) +Aβ25-35 (40 μM) group for 24 h; scale bar: 100 μm. E) PCMT1, Bax, Bcl 2, Caspase 3, Cleaved caspase 3, T-MST1 and p-MST1 protein expressions were evaluated by western blot in control, NC, sh-PCMT1 (48 h), sh-PCMT1 (48 h)+NCS-CDM, vector, PCMT1-OV (48 h), Aβ25-35 (40 μM) and PCMT1-OV (48 h) +Aβ25-35 (40 μM) group for 24 h.*P<0.05, **P<0.01, ***P<0.001 statistical significance.

Discussion

The purpose of the current study was to evaluate whether NSC-CDM regulates PCMT1 expression and inhibits Aβ25-35- induced toxicity in SH-SY5Y cells through mediation of the PCMT1/MST1 pathway.

Neurotoxicity in AD results from enhanced cellular processing of the amyloid precursor protein, interactions of Aβ with cell membranes, generation of reactive oxygen species, abnormalities in the protective response to oxidative stress, and/or susceptibility to apoptotic stimuli.15-17 Therefore, we aimed to assess Aβ25-35- induced apoptosis. In the present study, Aβ25-35 significantly decreased cell viability and induced apoptosis in SH-SY5Y cells. Traditionally, NSC-CDM has been discarded as waste as NSCs produce potentially harmful materials during cell growth in vitro. However, NSC-CDM has received more attention in recent years due to the observation of bystander actions of NSCs in vivo, especially in microvesicles released by NSCs. NSC-CDM has been shown to exert antiapoptotic effects in vitro18 and in vivo.13 The present study demonstrates that Aβ25-35 inhibits the cell viability and induces apoptosis in SH-SY5Y cells, which was reversed by treatment with NSC-CDM.

Apoptosis is regulated by intracellular apoptosis proteins, which are divided into pro-apoptotic and anti-apoptotic groups.[19] Bax is a pro-apoptotic protein, while Bcl-2 is an anti-apoptotic protein that prevents cytochrome c release from the mitochondria. The Bax and Bcl-2 levels are directly involved in the regulation of apoptosis. Bax is a component of an ion channel on the mitochondrial membrane and upregulation of Bax allows release of cytochrome c from the mitochondria. This in turn activates caspase 9, which leads to caspase 3 activation and apoptosis.[20] Reduced expression of Bcl-2 prevents it from interfering with cytochrome c release, leading to the activation of caspases and apoptotic proteins, thus promoting apoptosis.[21]

PCMT1 is highly expressed in brain tissue, including the substantia nigra, blue plaque, and paraventricular nucleus[7] and has been associated with apoptosis. For example, Sambri et al.[22] found that microRNA 15a/16-1 regulates apoptosis in hepatocellular tumor cells through PCMT1 and Liang et al.[10] showed that CGP3466B plays a neuroprotective role by regulating apoptosis through the PCMT1/MST1 pathway in rats with traumatic brain injury. MST1 is a silk/susine protein kinase, and is an important factor in the Hippo signaling pathway. MST1 is also involved in tumor occurrence and overgrowth of cells,[23] as MST1 deficiency can cause tumor formation which is associated with apoptosis of multiple cell types.[24] Overexpression of PCMT1 has been shown to prevent apoptosis induced by Bax in neurons and COS-1 cells. Overexpression of PCMT1 has also been shown to block H2O2- induced apoptosis through regulation of the methylation of various anti-apoptotic proteins, including Hsp90, Hsp70, and Bcl-xL, in vascular endothelial cells.25-28 Increased PCMT1 activity is also associated with neuroprotection due to prevention of neuronal apoptosis and cerebral edema.[10,11]

In the present study, a decrease in PCMT1 expression levels was observed in the Aβ25-35 -treated group. However, these effects were ameliorated by treatment with NSC-CDM. CCK-8 analysis showed that after knockdown of PCMT1 using sh-PCMT1, there was significantly reduced cell viability. Overexpression of PCMT1 resulted in increased cell viability and reversal of the effects of Aβ25-35 treatment. In addition, using a TUNEL assay, we found that the number of apoptotic cells were markedly higher in SH-SY5Y cells with sh-PCMT1 treatment, whereas, overexpression of PCMT1 reversed the effect of Aβ25-35 induced apoptosis. Western blotting showed that sh-PCMT1 and Aβ25-35 treatment significantly reduced PCMT1 expression, reduced the ratio of Bcl 2/Bax, increased the ratio of cleaved caspase 3/caspase 3, and increased the ratio of p-MST1/T-MST1. sh-PCMT1 inhibited the protective effects of NSC-CDM while overexpression of PCMT1 reversed these effects.

In summary, NSC-CDM corrects the damage of Aβ25-35 to SHSY5Y cells through increasing levels of PCMT1, thus reducing phosphorylation of MST.