Background
Glomerulonephritis is one of the main causes of end-stage renal disease (ESRD) all over the world. The incidence of glomerulonephritis ranges from 10.5% to 38.2% and its prevalence is 17.6%-53.5% [
1]. Mesangial proliferative glomerulonephritis (MsPGN) is characterized by the increased number of cells and extracellular matrix (ECM) in the mesangial region of glomerulus [
2]. The proliferation of mesangial cells can release inflammatory mediators to induce interstitial fibrosis and irreversible progressive glomerulosclerosis, eventually evolving into ESRD [
3]. The proliferation and inflammation of mesangial cells play an important role in the development of MsPGN [
4]. Therefore, reducing the proliferation and inflammation of mesangial cells is of great significance for improving the therapeutic effect of MsPGN. In recent years, research on the molecular mechanisms of human diseases has provided more potential therapeutic targets [
5,
6]. Exploring the molecular mechanism of MsPGN development is conductive to finding effective targets for alleviating the proliferation and inflammation of mesangial cells.
Neurotrophic tropomyosin receptor kinase 1 (Ntrk1), located at 1q21-q22, belongs to the tyrosine receptor kinase (Trk) family. The protein encoded by Ntrk1 is also called tyrosine receptor kinase A (TrkA) [
7]. Previous study has discovered that TrkA expression is up-regulated in kidney tissues of patients with diabetic nephropathy (DN). However, in healthy human kidney tissues, TrkA is only expressed at a low level. Animal experiment indicates that inhibition of TrkA may be conductive to preventing kidney fibrosis [
8]. Bonofiglio et al
. [
9] discovered that in kidney biopsy samples from patients with renal diseases, TrkA expression was detected in tubular and glomerular cells. In addition, TrkA was over-expressed in the peripheral blood mononuclear cells (PMNCs) in glomerulonephritis patients [
10]. However, the exact role of Ntrk1 in MsPGN and the underlying mechanism have not been elucidated yet. Thus, in this study, the MsPGN rat model was constructed. The exact expression and effect of Ntrk1 on the proliferation and inflammation of mesangial cells were investigated. This article can provide a novel therapeutic target for MsPGN.
Methods
Ethics committee
The animal experiment in this study was approved by the Animal Ethics Committee of The Second People's Hospital of Wuhu.
Construction of the MsPGN rat model
Twelve male SD rats weighing 200–220 g were purchased from Shanghai Laboratory Animal Center, Chinese Academy of Sciences. All rats had free access to food and water, and were kept in a room under 22 °C and 12-h/12-h light/dark cycle conditions. Nine rats were used to construct the MsPGN model by single injection of Thy1.1 monoclonal antibody (2.5 mg/kg, ab95812, Abcam, Shanghai, China) via the tail vein [
11]. The Thy1.1 monoclonal antibody was generated by OX-7 cells. At 3, 7 and 14 days after injection, three rats were euthanized to obtain kidney tissues. The glomeruli were isolated as described previously [
12]. Furthermore, the other three rats were given injection of an equivalent volume of normal saline and then euthanized on day 0 to collect kidney tissues and glomeruli. The kidney tissues and glomeruli were later stored at -80 °C.
Rats were euthanized as follows: Rats were deeply anesthetized by inhalation of 5% isoflurane [
13]. When rats had no response to head and limb stimulation, they were rapidly executed by cervical dislocation. After 10 s of cervical dislocation, rats were identified as dead if they stopped breathing and had no respond to systemic stimulation [
14].
Construction of the Ntrk1 knockdown rat model
Male SD rats weighing 200–220 g (
n = 36, Shanghai Laboratory Animal Center, Chinese Academy of Sciences) were randomly divided into three groups, including PBS group (
n = 12), shNC group (
n = 12) and shNtrk1 group (
n = 12). Rats of each group had free access to water and food and raised in a room under 22 °C and 12-h/12-h light/dark cycle conditions. Rats of PBS group, shNC group and shNtrk1 group were respectively injected with 0.2 mL phosphate buffered solution (PBS), 0.2 mL negative control lentivirus (1 × 10
9 TU/mL) and Ntrk1-RNAi lentivirus (1 × 10
9 TU/mL) via tail vein thrice at 5-day intervals [
15]. Afterwards, three rats from each group were randomly selected for euthanasia on days 0, 3, 7 and 14. The kidney tissues were obtained, the glomeruli were isolated, and all of them were stored at -80 °C.
Male SD rats weighing 200–220 g (n = 9, Shanghai Laboratory Animal Center, Chinese Academy of Sciences) were randomly divided into three groups, namely, Sham group (n = 3), shNC + Thy-1 GN group (n = 3) and shNtrk1 + Thy-1 GN group (n = 3). All rats had free access to water and food, and were raised in a room under 22 °C and 12-h/12-h light/dark cycle conditions. Rats of Sham group did not receive any treatment, while those of shNC + Thy-1 GN group and shNtrk1 + Thy-1 GN group were given injection of negative control lentivirus (1 × 109 TU/mL) and Ntrk1-RNAi lentivirus (1 × 109 TU/mL), respectively, via the tail vein thrice at 5-days intervals. Thereafter, rats from the two groups were subject to injection of Thy1.1 monoclonal antibody (2.5 mg/kg) via the tail vein. On day 7 of Thy1.1 monoclonal antibody injection, rats of each group were euthanized. Afterwards, kidney tissues were obtained, glomeruli were isolated, and all of them were stored at -80 °C.
Periodic acid-schiff (PAS) staining
The rat kidney tissues were fixed with 10% formaldehyde solution, embedded in paraffin, and prepared into 4-μm sections. After xylene deparaffinage, the sections were rehydrated with gradient ethanol and stained with PAS. Hematoxylin was later utilized for the counter-staining of sections. The total cell number in each glomerulus cross-section was counted under a microscope. For each rat, cells were counted in three consecutive sections of 10 glomeruli. Simultaneously, the mesangial matrix score was evaluated to determine the glomerulus damage degree. The scoring standard was as follows: 0 point, normal; 1 point, slight glomerulus damage of mesangial matrix and hyalinosis with focal adhesion involving < 25% of the glomerulus; 2 points, 25%-50% of glomerulus sclerosis; 3 points, 50%-75% of sclerosis; and 4 points, > 75% of sclerosis [
16]. The experiment was independently repeated thrice.
Immunohistochemistry (IHC) analysis
The 4-μm kidney tissue sections were deparaffinized with xylene, rehydrated with an ascending series of gradient ethanol, and blocked with 3% H
2O
2. Thereafter, the sections were incubated with sodium citrate buffer (pH = 6.0) for 10 min and later with goat serum (5%) for 20 min. After washing, the sections were incubated with rabbit anti-Ki67 primary antibody (1:100, ab92742, Abcam, Cambridge, MA, USA) for 12 h at 4 °C. Afterwards, the unbound primary antibody was washed with PBS, and the sections were further incubated with biotinylated secondary antibody for 40 min at room temperature. Thereafter, sections were treated with horseradish peroxidase (HRP)-streptavidin working buffer for 40 min at room temperature, and later stained with 3,3'-diaminobenzidine (DAB) and hematoxylin sequentially. The number of Ki67-positive cells for each rat was counted from three consecutive sections under a microscope [
17]. The experiment was independently repeated three times.
Detection of urinary protein
The rat urine samples were collected over a 24-h period using metabolic cages on days 0, 3, 7 and 14 after injection. Thereafter, each urine sample was centrifuged for 5 min at 14,000 rpm [
18]. The urine protein in the supernatant was detected using an urine protein assay kit (Pierce, Rockford, IL, USA) according to specific instructions. The experiment was independently repeated three times.
Culture and transfection of mesangial cells
The rat mesangial cell line was commercially provided by the American Type Culture Collection (ATCC, Rockville, MD, USA) and maintained in the RPMI-1640 medium supplemented with 10% fetal bovine serum (FBS) at 37 °C and 5% CO2 conditions.
After reaching 85% confluence, mesangial cells were harvested after trypsinization. Then, cells were prepared into the single cell suspension with serum-free RPMI-1640 medium at a cell concentration of 1 × 105 cells/mL. Thereafter, 1 mL cell suspension was added into each well of the 6-well plates. shRNA targeting Ntrk1 and corresponding negative control, pcDNA-Ntrk1 vector and empty vector were purchased from Genechem (Shanghai, China), and transfected into mesangial cells using Lipofectamine 2000 (Thermo Fisher Scientific, Waltham, MA, USA) in strict accordance with specific instructions. Cells were sequentially grouped as shNtrk1 group, shNC group, Ntrk1 group and Vector group. After 8 h of transfection, cells were cultured in RPMI-1640 medium supplemented with 10% FBS for 48 h.
Treatment of mesangial cells with p38 inhibitor and ERK inhibitor
p38 inhibitor (SB202190) and ERK inhibitor (PD98059) were utilized respectively to treat mesangial cells. Briefly, the pcDNA-Ntrk1 vector-transfected mesangial cells were cultured with RPMI-1640 medium containing 10% FBS and 1 µM [
19] SB202190 (VB2726-10, AmyJet Scientific, Wuhan, China) or 10 µM [
20] PD98059 (S1805, Beyotime, Shanghai, China) at 37 °C and 5% CO
2 conditions.
Cell counting kit-8 (CCK-8) assay
The cell viability of mesangial cells was investigated using the CCK-8 assay. In brief, mesangial cells were prepared into the single cell suspension with RPMI-1640 medium containing 10% FBS at a cell density of 1 × 105 cells/mL. Thereafter, 100 μL cell suspension was seeded into each well of the 96-well plates. Three replicate wells were set for each group, and the plates were placed under 37 °C and 5% CO2 conditions for 48 h. Subsequently, 10 μL CCK-8 reagent (Beyotime, Shanghai, China) was added into each well to incubate cells for 4 h at 37 °C. The optical density (OD) of each well was measured at 450 nm using a microplate reader. The experiment was independently repeated thrice.
The 5-Ethynyl-2′-deoxyuridine (EdU) assay
The proliferation of mesangial cells was investigated by the EdU assay. Cells were dispersed into RPMI-1640 medium containing 10% FBS (1 × 105 cells/mL) and seeded in the 6-well plates for 48 h under 37 °C and 5% CO2 conditions. Thereafter, 1 mL cell suspension was added into each well. Thereafter, cell proliferation was investigated using an EdU immunofluorescence assay in line with specific instructions (Beyotime, Shanghai, China). EdU-positive cells presented with red fluorescence, meanwhile, the nucleus was stained with blue fluorescence by using 4',6-diamidino-2-phenylindole (DAPI). The EdU-positive cells were counted under a fluorescence microscope from five random non-overlapping fields of view (FOVs). The experiment was independently repeated three times.
Quantitative real-time polymerase chain reaction (qRT-PCR)
Total RNAs in glomeruli and mesangial cells were extracted with TRIzol reagent (Solarbio, Beijing, China) strictly in line with specific instructions. Thereafter, 3 µg total RNA samples were collected for the synthesis of cDNA template by reverse transcription using a reverse transcription kit (Promega, Madison, WI, USA). qRT-PCR was carried out using SYBR Select Master Mix (Life Technologies, California, USA) and an RT-PCR detection system (Applied Biosystems, Foster City, CA, USA). The reaction procedure was as follows: 40 cycles of denaturation at 95 °C for 30 s, annealing at 55 °C for 30 s, and extension at 72 °C for 60 s. The following primers were used, for IFN-γ, sense: 5'-ACAACCCACAGATCCAGC-3', antisense: 5'-TCAGCACCGACTCCTTTT-3'; for TNF-α, sense: 5'-CCACGCTCTTCTGTCTACTG-3', antisense: 5'-GGGAACTTCTCCTCCTTGTT-3'; for IL-6, sense: 5'-GGAGTTCCGTTTCTACCT-3', antisense: 5'-CTCTGGCTTTGTCTTTCT-3'; for Ntrk1, sense: 5′-GGACAACCCTTTCGAGTTCA-3′, antisense: 5′-GTGGTGAACACAGGCATCAC-3′; for β-actin (control), sense: 5′-AGCGAGCATCCCCCAAAGTT-3′, antisense: 5′-GGGCACGAAGGCTCATCATT-3′. The relative expression of the other mRNAs was calculated by the 2−ΔΔCt method. The experiment was independently repeated three times.
Western-blot (WB) assay
Glomeruli and mesangial cells were lysed with RIPA buffer on ice for 30 min. The lysate was centrifuged at 15,000 rpm and 4 °C for 30 min. Thereafter, the supernatant was harvested and the concentration of total protein was detected with the BCA Protein Assay Kit (Beyotime, Shanghai, China). Then, 30 μg of each protein sample was collected, boiled for 5 min, and separated with 10% sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Afterwards, the separated protein was transferred onto the polyvinylidene fluoride (PVDF) membrane, which was later blocked with skimmed milk (5%) for 1 h at room temperature. Protein was later probed with primary antibodies (1:1000) overnight at 4 °C, including rabbit anti-Ntrk1 (1:1000, PAB12224, AmyJet Scientific, Wuhan, China), anti-STAT3 (1:1000, ab226942, Abcam, Shanghai, China), anti-pSTAT3 (1:1000, BY-1658R, Kemin Biotechnology, Shanghai, China), anti-p38 (1:1000, ab47363, Abcam, Shanghai, China), anti-pp38 (1:1000, YB063, Yubo Biotechnology, Shanghai, China), anti-ERK (1:1000, ab17942, Abcam, Shanghai, China), anti-pERK (1:1000, HK10249, Hushi Pharmaceutical Technology, Shanghai, China), anti-AKT (1:1000, ab89402, Abcam, Shanghai, China), anti-pAKT (1:1000, A-ABV10202, AmyJet Scientific, Wuhan, China), anti-JNK (1:1000, A-AP51547, AmyJet Scientific, Wuhan, China), anti-pJNK (1:1000, K009325P, Solarbio, Beijing, China) and anti-β-actin (1:1000, ab8227, Abcam, Shanghai, China). Subsequently, the PVDF membrane was washed twice with Tris-buffered saline containing 0.1% Tween 20 buffer, and further incubated with HRP-conjugated goat anti-rabbit secondary antibodies (1:2000, ab6721, Abcam, Shanghai, China) for 2 h at room temperature. The protein bands were then developed with the enhanced chemiluminescence (ECL) substrate (Beyotime, Shanghai, China) in the LI-COR imaging system (LI-COR Biosciences, Lincoln, NE, USA). Finally, the densitometry analysis of protein blots was conducted by ImageJ analysis system (NIH, Bethesda, MD, USA). The experiment was independently repeated thrice.
Statistical analysis
All data were obtained from three independent repeated experiments and expressed as mean ± standard deviation (SD). SPSS19.0 software was adopted for statistical analysis. Significant differences between two groups were analyzed by two-tailed paired Student's t-test. One-way analysis of variance (ANOVA) and post-hoc Tukey's test were used to analyze the differences among multiple groups. P < 0.05 was considered statistically significant.
Discussion
As the most active cells in the glomerulus, the proliferation of mesangial cells plays an important role in the development of MsPGN. Mesangial cells can respond to multiple external stimuli (such as macromolecular substances, immune complexes and hypoxia) via the proliferative reaction [
2,
21]. The main pathological feature of MsPGN includes the proliferation and expansion of mesangial cells [
22]. Hence, inhibiting mesangial cell proliferation is one of the main strategies for treating MsPGN. Previous study has reported that the injection of Thy1.1 monoclonal antibody through tail vein can successfully establish the rat MsPGN model [
11]. Therefore, this work constructed the rat MsPGN model through single injection of Thy1.1 monoclonal antibody (2.5 mg/kg) via the tail vein. On day 7 after the injection of Thy1.1 monoclonal antibody, mesangial cell proliferation and glomerulus inflammatory response reached a peak. Importantly, Ntrk1 mRNA and protein expression was abnormally up-regulated in the glomeruli of MsPGN rats. As revealed by studies in-vivo and in-vitro, Ntrk1 knockdown inhibited the proliferation of mesangial cells and the expression of pro-inflammatory factors (including IFN-γ, TNF-α and IL-6) in glomeruli. Meanwhile, Ntrk1 knockdown reduced the urinary protein level in MsPGN rats. As far as we know, this is the first time that Ntrk1 is identified to promote the development of MsPGN by enhancing mesangial cell proliferation and inflammatory response.
After Ntrk1 knockdown, the activity of the STAT3 signaling pathway was reduced in glomeruli of MsPGN rats and mesangial cells. Meanwhile, Ntrk1 knockdown alleviated glomerulus injury in MsPGN rats. Previous study has reported that the activation of STAT3 signaling pathway is an important factor to induce the progression of glomerulonephritis. The level of phosphorylated STAT3 (pSTAT3) is consistent with the peak of mesangial cell proliferation in glomeruli. Interestingly, the expression of pSTAT3 decreased with the attenuation of glomerular proliferation and sclerosis. Thus, the activation of the STAT3 signaling pathway enhances the progression of glomerulus sclerosis in glomerulonephritis [
23]. Meanwhile, the activation of the STAT3 signaling pathway is demonstrated to be critical for the development of crescentic glomerulonephritis, which exacerbates the proliferation of glomerular cells and the damage of kidney [
24]. Therefore, in this work, Ntrk1 might promote the viability, proliferation and inflammatory response of mesangial cells by stimulating the activation of the STAT3 signaling pathway.
p38 is one of the main members of the MAPK signaling pathway, and its activation can lead to the proliferation of mesangial cells in diabetic nephropathy [
25]. Zhu et al
. [
26] revealed that the activation of the p38 signaling pathway was necessary for the development of MsPGN in rats. As another member of the MAPK family, ERK is the core of the signaling network that regulates cell growth and division [
27]. Leonard et al
. [
28] illustrated that the activation of p38 and ERK signaling pathways is crucial for the production of IL-6 in mesangial regions of glomeruli. Simultaneously, the activation of ERK signaling pathway is demonstrated to aggravate the proliferation of mesangial cells in the rat MsPGN model [
29]. In lupus nephritis, the expression of phosphorylated ERK (pERK) is reduced by miR-155, which significantly inhibits the proliferation of mesangial cells [
30]. The activation of ERK signaling pathway is reported to be involved in the inflammatory response. Sun et al
. [
31] found that the inactivation of ERK signaling pathway alleviated the inflammatory response. In this study, it was demonstrated that Ntrk1 knockdown suppressed the phosphorylation of p38 and ERK. On the opposite, Ntrk1 up-regulation promoted the phosphorylation of p38 and ERK. More interestingly, treatment with p38 inhibitor and ERK inhibitor reversed the effect of Ntrk1 on promoting the viability, proliferation and inflammatory response of mesangial cells. As suggested by in-vivo study, Ntrk1 knockdown alleviated glomerulus injury in MsPGN rats, and suppressed the activity of p38 and ERK signaling pathways. Taken together, Ntrk1 might promote the viability, proliferation and inflammatory response of mesangial cells by stimulating the activation of p38 and ERK signaling pathways.
In previous studies, the AKT signaling pathway has been found to be activated in mesangial cells, which represents a key mechanism for promoting the proliferation and inflammatory response of mesangial cells in rats with MsPGN [
2,
21,
32]. Moreover, JNK is a crucial member of the MAPK signaling pathway [
33]. The repressed activity of the JNK signaling pathway is implied to attenuate the proliferation of mesangial cells, so as to relieve renal lesions in IgA nephropathy [
34]. Intriguingly, this study implied that Ntrk1 did not significantly affect the activation of AKT and JNK signaling pathways. The underlying mechanism of this phenomenon will be investigated in our future research.
Certain limitations should be noted in this paper. Previous studies have implied that there is crosstalk between the STAT3 and MAPK signaling pathways [
35‐
37]. Therefore, it will be better to verify whether there is an upstream and downstream regulatory relationship between the STAT3 and MAPK signaling pathways. Moreover, this paper indicated that Ntrk1 overexpression or knockdown did not obviously affect the activation of AKT and JNK signaling pathways in mesangial cells. It should be better to conduct a more in-depth study on the underlying mechanism of this phenomenon. However, due to the limitations of laboratory conditions, these issues cannot be currently verified. We will devote ourselves to investigating these interesting issues in the future research.
In summary, this paper established a rat MsPGN model and explored the effect of Ntrk1 on the proliferation and inflammatory response of mesangial cells. The results revealed that Ntrk1 was abnormally over-expressed in glomeruli of MsPGN rats. Ntrk1 knockdown reduced the proliferation and inflammation of mesangial cells, whereas Ntrk1 overexpression showed the opposite effect. In terms of the underlyign mechanism, Ntrk1 might promote the proliferation and inflammation of mesangial cells via activating the STAT3, p38 and ERK signaling pathways. Thus, Ntrk1 might be a promising therapeutic target for MsPGN.
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