Background
The inhibitory microenvironment composed of an inflammatory response in the acute phase and scar tissue formation in the chronic phase is considered to be the main reason that hinders axon regeneration after spinal cord injury (SCI) [
1,
2]. Because previous studies mostly focused on astrocytic scar formed by astrocytes after SCI, fibrotic scar formed by fibroblasts is less well understood [
2,
3]. Following SCI, perivascular fibroblasts leave the blood vessel, proliferate, migrate and deposit fibrous extracellular matrix (ECM), including fibronectin, laminin and collagen, finally forming fibrotic scar inside the astrocytic scar, which hinders axon regeneration [
4‐
7]. Moderate inhibition of fibrotic scar formation contributes to axon regeneration and locomotor function recovery, indicating that the adverse effects of excessively deposited fibrotic scar are greater than the beneficial effects after SCI [
6,
8]. These results reveal that the role of fibrotic scar in the targeted therapy of SCI is of great significance and should be given more attention. However, the molecular mechanism of fibrotic scar formation after SCI is unclear.
Platelet-derived growth factors (PDGFs) are a cysteine-knot-type growth factor family composed of four polypeptide chains A, B, C, and D [
9]. These growth factors activate intracellular signalling by binding to platelet-derived growth factor receptor (PDGFR) α or PDGFRβ, while PDGFRβ can only be activated by PDGFB and PDGFD and plays an important role in cell proliferation, differentiation and migration [
10‐
12]. A large number of studies have shown that the PDGF/PDGFR pathway is a critical functional mediator of neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD) and amyotrophic lateral sclerosis (ALS), while relatively little is known about it in SCI [
12]. It has been reported that PDGFRβ, as a marker of perivascular fibroblasts, is expressed in almost all scar-forming fibroblasts [
5,
6,
13], while whether the PDGFRβ pathway is involved in fibrotic scar formation after SCI is still lacking direct evidence. Our previous in vitro study showed that PDGFB can regulate the migration of fibrotic scar-forming cells model PDGFRβ
+ pericytes/fibroblasts, which can be aborted by the PDGFRβ inhibitor SU16f [
14]. Nevertheless, the cellular location and function of the PDGF/PDGFRβ pathway after SCI need to be further explored in vivo.
In this study, our results showed that the expression of PDGFD occurred earlier than that of PDGFB after SCI, and PDGFB was mainly secreted by astrocytes, while PDGFD was mainly secreted by macrophages/microglia and fibroblasts. Intrathecal injection of the PDGFRβ inhibitor SU16f blocked the fibrosis induced by exogenous PDGFB or PDGFD in the uninjured spinal cord. In addition, SU16f blockade of the PDGFRβ pathway resulted in the reduction and interruption of fibrotic scar and the resolution of lesion and inflammation, thereby facilitating axon regeneration and locomotor function recovery after SCI. These results indicate that the PDGFRβ pathway is essential for fibrotic scar formation after SCI and is expected to be a therapeutic target for SCI.
Materials and methods
Animals and spinal cord compression injury model
All experiments involving animals were approved by the Ethics Committee of Anhui Medical University (Approval No. LLSC20160052). Eight-week-old C57BL/6 mice were acquired from the Animal Experiment Center of Anhui Medical University and were housed in an environment with controlled temperature and humidity and a 12:12 h light:dark cycle. The animals were randomly grouped and kept in standardized cages, where water and food were readily available.
The establishment of the spinal cord compression injury model has been described in detail in our previous study [
14]. In brief, after satisfactory anaesthesia with isoflurane (induction 4%, maintenance 2%), the mid-thoracic level (T10) spinal cord was carefully exposed and compressed with calibrated Dumont #5 forceps (11252-20, Fine Science Tools, Germany) for 5 s. The postoperative mice received anti-infection treatment and auxiliary urination nursing twice a day.
In situ injection of PDGFB or PDGFD
The object of in situ injection of PDGFB was the uninjured spinal cord of mice. The T10 spinal cord was exposed according to the established method of the spinal cord injury model, and then the mouse was fixed on the stereotaxic device. The insertion site of the microinjection needle (7634-01 and 7803-05, Hamilton, Switzerland) was 0.3 mm lateral to the midline and 0.8 mm deep to the dorsal surface of the mouse spinal cord [
15]. Two microlitres of 100 ng/μl recombinant human PDGFB (HZ-1308, Proteintech, China) dissolved in 10 mM HOAc containing 0.1% bovine serum albumin (BSA) or PDGFD (1159-SB/CF, R&D Systems, United States) dissolved in 4 mM HCl containing 0.1% BSA was injected into the uninjured spinal cord at 0.5 μl/min using a stereotaxic injector (KDS LEGATO 130, RWD, China). The control mice received 2 μl of 10 mM HOAc containing 0.1% BSA or 4 mM HCl containing 0.1% BSA. All mice were sacrificed at 7 days after injection.
Intrathecal injection of SU16f
The needle insertion site was located in the dorsal midpoint of the lumbar 5–6 intervertebral space as previously reported [
16]. It was confirmed that the needle was successfully inserted into the intradural space by observing an evident sudden tail flick. Ten microlitres of 3 mM SU16f dissolved in 0.1 M phosphate buffered saline (PBS) containing 3% DMSO (3304, R&D Systems, United States) was injected daily at 1 μl/4 s using a microinjection needle (1701, Hamilton, Switzerland). For the mice without SCI, SU16f was preinjected the day before the injection of PDGFB or PDGFD and then injected daily for 7 consecutive days from the day of PDGFB or PDGFD injection. For the mice with SCI, SU16f was injected from 3 day post-injury (dpi) until sacrifice. The control mice received 10 μl of PBS containing 3% DMSO.
Intraperitoneal injection of Bromodeoxyuridine (BrdU)
To label proliferating fibroblasts, mice received intraperitoneal injection of 50 mg/kg body weight BrdU (BS916, Biosharp, China) daily for 1–6 dpi. All mice were sacrificed at 7 dpi.
Tissue preparation and immunofluorescent staining
After cardiac perfusion with 0.1 M PBS (Servicebio, China) followed by 4% paraformaldehyde (PFA, Servicebio, China), the 0.5 mm segment of spinal cord tissue containing the injured core was placed in 4% PFA and postfixed for 5 h. The tissue was then placed in a 30% sucrose solution and dehydrated at 4 °C for 24 h until the tissue sank to the bottom. Finally, the tissue was cut into 18 μm-thick serial sagittal or coronal sections using a cryostat (NX50, Thermo Fisher Scientific, United States). The sections encompassing the lesion core or injection site were used.
For BrdU staining, the sections were pretreated with 2 N hydrochloric acid (HCl, GEMIC, China) at 37 °C for 30 min followed by 0.1 M borate buffer (KGR0101, KeyGEN BioTECH, China) at room temperature for 10 min and were subjected to an immunofluorescence staining protocol. The sections were blocked in 10% donkey serum containing 0.3% Triton X-100 (SL050 and T8200, Solarbio, China) at room temperature for 1 h, followed by incubation with primary antibodies at 4 °C overnight. The primary antibodies included goat anti-PDGFRβ (5 μg/ml, AF1042-SP, R&D Systems, United States), goat anti-CD31 (1:100, AF3628, R&D Systems, United States), goat anti-5-hydroxytryptamine (5-HT) (1:5000, 20079, Immunostar, United States), rabbit anti-PDGFB (1:100, NBP1-58279, Novus, United States), rabbit anti-PDGFD (1:100, 40–2100, Thermo Fisher Scientific, United States), rabbit anti-fibronectin (1:100, 15613-1-AP, Proteintech, China), rabbit anti-laminin (1:100, 23498-1-AP, Proteintech, China), rabbit anti-neurofilament (NF) (1:500, ab207176, Abcam, United States), rat anti-GFAP (1:400, 13-0300, Thermo Fisher Scientific, United States), rat anti-CD68 (1:400, MCA1957, Bio-Rad, United States), rat anti-BrdU (1:200, ab6326, Abcam, United States) and rat anti-Ki67 (1:100, 14-5698-80, Thermo Fisher Scientific, United States). Subsequently, the sections were incubated with appropriate secondary antibodies at room temperature for 1 h, including donkey anti-goat Alexa Fluor 488, donkey anti-goat Alexa Fluor 555, donkey anti-goat Alexa Fluor 647, donkey anti-rabbit Alexa Fluor 555, donkey anti-rat Alexa Fluor 488 and donkey anti-rat Alexa Fluor 555 (1:500, A-11055, A-21432, A-21447, A-31572, A-21208, A48270, Thermo Fisher Scientific, United States). Finally, the sections were stained with DAPI (C1005, Beyotime Biotechnology, China) to label the nuclei. The negative control sections were incubated with secondary antibody alone.
Image acquisition and quantitative analysis
Representative images of the sections were acquired using a Zeiss LSM 900 confocal microscope system and a Zeiss Axio Scope A1 fluorescence microscope. Staining colocalization was determined using ZEN 3.3 software to examine each of the ten one-micron Z-stack slices. Image processing was performed using ImageJ version 2.0 (NIH, United States).
All quantitative analyses were performed in a blind fashion. To quantify GFAP
+, CD68
+, CD31
+, PDGFRβ
+, PDGFB
+ and PDGFD
+ cells, 100 μm square grids were generated over the injured site [
17]. Every 6th square was quantified, and only DAPI
+ cells were counted. One section encompassing the lesion core in each sample was used for counting, with 5 samples per group.
To evaluate the area of fibrotic scar, the immunoreactivities of PDGFRβ, fibronectin and laminin were normalized to the area of the spinal cord segment spanning the injured core in a 4 × image [
17]. Similarly, the GFAP
− area and CD68
+ area was normalized to the area of the spinal cord segment spanning the injured core in a 4 × image. To evaluate axon regeneration, the immunoreactivity of 5-HT was normalized to the area of the spinal cord segment spanning the injured core in a 10 × image, and the number of NF
+ axons longer than 1 μm in the GFAP
− region was counted and normalized to the area of the GFAP
− region. For each sample, sections spanning the injured core and two adjacent sections spaced 180 μm apart were quantified, and the results from each section were averaged, with 5 samples per group.
To evaluate the proliferation of fibroblasts, BrdU+ PDGFRβ+ or Ki67+ PDGFRβ+ cells were counted on 40 × images spanning the injured core. The average of three random 40 × images was used as the final result of each sample, with 5 samples per group.
Behavioural assessments
The Basso Mouse Scale (BMS) is widely used to evaluate locomotor function recovery after SCI in mice [
18]. In this study, BMS was performed in an open field according to the protocol developed by Basso and colleagues [
19]. All mice received BMS to confirm normal locomotor function before SCI and received BMS to confirm the success of the SCI model after surgery. Each mouse was assessed by two experienced examiners at 3, 7, 14, 21 and 28 dpi, and the average value was finally obtained, with 8 animals per group.
Footprint analysis was used to further evaluate locomotor function recovery at 28 dpi and was performed according to previous reports [
20]. The mice without SCI received footprint analysis were included in the uninjured group. The front paws were dipped in green dyes, and the hind paws were dipped in red dyes. The stride length was determined by the distance from the beginning to the end of the hind paw in a step. The stride width was determined by the distance from the outermost toe of the left paw to the outermost toe of the right paw. The paw rotation was determined by the angle between the midline axis of the body and the axis of the hind paw. All assessments were performed in three consecutive gait cycles on each side and averaged, with 8 animals per group.
All behavioural assessments were performed in a blind fashion.
Statistical analysis
The data are presented as the mean ± standard error of the mean (SEM), and individual data points are plotted in the figures. The statistical methods used are presented in the figure legends. Multiple comparisons were analysed with one-way or two-way analysis of variance (ANOVA) with a post hoc Tukey–Kramer test, and comparisons between two groups were performed using Student’s t test. Data analysis and chart production were performed using GraphPad Prism 8.0 (GraphPad, United States), and a value of p < 0.05 was considered statistically significant.
Discussion
In this study, we found that the expression of PDGFD occurred earlier than that of PDGFB after SCI, and PDGFB was mainly secreted by astrocytes, while PDGFD was mainly secreted by macrophages/microglia and fibroblasts. Moreover, in situ injection of exogenous PDGFB or PDGFD can lead to fibrosis in the uninjured spinal cord, while SU16f blockade of the PDGFRβ pathway reduced the fibrotic scar area, interrupted the fibrotic/astrocytic scar boundary, shrunk the lesion and inhibited inflammation, promoting axon regeneration and locomotor function recovery after SCI. Therefore, the PDGFRβ pathway is expected to be a therapeutic target after SCI.
SCI is a devastating trauma and causes sensory and locomotor dysfunction in patients, and there is currently a lack of effective clinical treatments [
25,
26]. Therefore, it is of great significance to explore the pathological changes and molecular mechanisms of SCI, so as to provide new ideas for treatment. Scars, as one of the critical factors hindering axon regeneration after SCI, mainly include fibrotic scar formed by fibroblasts and astrocytic scar formed by astrocytes [
1,
5]. Although the deposition of chondroitin sulphate proteoglycans (CSPGs) by astrocytes leads to the failure of axon regeneration after SCI [
27,
28], the secretion of axon-growth-supporting molecules by astrocytes is required for axon regeneration and astrocytic scar has the beneficial effect of limiting inflammation [
29,
30]. Meanwhile, recent studies have shown that inhibition of astrocytic scar formation cannot promote axon regeneration, while astrocytic scar formation aids rather than prevents axon regeneration [
29,
31], suggesting that the beneficial effects of astrocytic scar in SCI are greater than the adverse effects. However, moderate inhibition of fibrotic scar formation can promote axon regeneration and functional recovery after SCI [
6,
17], indicating that fibrotic scar is of great significance as a therapeutic target for SCI.
Following SCI, perivascular fibroblasts leave blood vessels, proliferate and migrate to the injured site at 3–7 dpi [
4,
5]. At 7–14 dpi, fibroblasts deposit large amounts of fibrous ECM, including fibronectin, laminin and collagen, to form fibrotic scar that corrals macrophages in the injured core and is located on the inner side of astrocytic scar [
7,
17,
18]. It has been accepted that fibrotic scar significantly hinders axon regeneration after SCI [
8,
32‐
34]. The Jonas Frisén group used Glast–Rasless transgenic mice to specifically block the proliferation of fibroblasts after SCI, thereby establishing a fibrotic scar removal model [
4,
6]. In addition, they revealed that complete elimination of fibrotic scar leads to the failure of injured site closure and the spread of inflammation after SCI, while a moderate reduction in fibrotic scar inhibits inflammation and promotes axon regeneration after SCI [
4,
6], suggesting that fibrotic scar can be used as a therapeutic target after SCI. However, previous reports mainly used transgenic strategies or nonspecific target intervention strategies to inhibit fibrotic scar formation after SCI [
6,
8,
32]. For instance, transforming growth factor beta (TGF-β) not only is a profibrotic factor but also participates in a variety of biological processes [
35,
36]. Administration of 8-Br-cAMP, Taxol, epothilone B (epoB) or antagomir-21 has been successfully used to suppress fibrotic scar after SCI via inhibiting TGFβ pathway, while targeting TGFβ is not specific for regulating fibrotic scar [
32‐
34,
37]. Therefore, a better understanding of the molecular mechanism of fibrotic scar formation after SCI could lead to the uncovering of specific molecular therapeutic targets, which is of major significance.
PDGFRβ is a transmembrane receptor tyrosine kinase composed of an intracellular tyrosine kinase domain and an extracellular ligand binding domain [
9]. PDGFB or PDGFD binding to PDGFRβ monomers mediates dimerization of PDGFRβ monomers and then activates kinase activity to trigger intracellular signalling cascades, including Janus kinase (JAK), phospholipase C gamma (PLCγ) and phosphoinositide 3-kinase (PI3K), involved in cell proliferation, differentiation and migration [
10,
11]. It has been reported that PDGFRβ is expressed in pericytes, astrocytes, NG2 cells and endothelial cells in brain injury models [
38]. However, PDGFRβ is widely used to label pericytes and regulates the survival, proliferation and migration of pericytes in the brain, thereby participating in angiogenesis and the repair and maintenance of the blood–brain barrier (BBB) [
12]. In the injured spinal cord, PDGFRβ is expressed on all fibrotic scar-forming fibroblasts, and fibroblasts account for up to 95% of PDGFRβ
+ cells, indicating that PDGFRβ is specifically expressed in fibroblasts after SCI [
7,
8,
13]. However, the spatiotemporal distribution of the ligands PDGFB and PDGFD and the effect of activation of the PDGFRβ pathway on fibroblasts forming fibrotic scar after SCI remain unclear. In this study, we found that PDGFB and PDGFD were highly expressed and distributed adjacent to PDGFRβ after SCI, suggesting that PDGFB or PDGFD may activate PDGFRβ to be involved in the formation of fibrotic scar after SCI. In addition, PDGFB began to be expressed in a large area from 7 dpi and was gradually distributed around the lesion epicentre, while PDGFD began to be expressed in a large area from 3 dpi and was gradually distributed at the lesion epicentre. These results indicate that PDGFD may be mainly involved in the recruitment and proliferation of fibroblasts in the early stage, while PDGFB may be mainly involved in the assembly and maturation of fibrotic scar in the late stage. The functional difference between PDGFB and PDGFD needs to be further studied, which is expected to provide a theoretical basis for sequential intervention of the PDGFRβ pathway after SCI.
Fibroblasts, astrocytes, vascular endothelial cells and macrophages/microglia are important cellular components at the injured site of SCI, and recent evidence has demonstrated extensive crosstalk among them [
17,
18,
39,
40]. The inhibition of fibrotic scarring results in the attenuation of astrogliosis and the interruption of astrocytic scar boundary after SCI [
6]. Besides, the inhibition of astrocytic scarring leads to the interruption of fibrotic scar boundary and the spread of inflammation after SCI [
39,
41], and the depletion of macrophages or microglia in the injured core leads to the interruption of fibrotic scar boundary after SCI [
17,
18,
42]. However, the molecular mechanism cues for the crosstalk among the cells remain largely elusive. Therefore, we further investigated the cell sources of PDGFB and PDGFD and focused the sources on macrophages, astrocytes, vascular endothelial cells or fibroblasts, which was expected to provide a basis for the crosstalk among the main cell components at the injured site. Our results showed that PDGFB was mainly secreted by astrocytes, while PDGFD was mainly secreted by macrophages/microglia and fibroblasts after SCI. The different sources of PDGFB and PDGFD indicate their different functions after SCI, while whether the PDGF/PDGFRβ pathway plays a role in the crosstalk among astrocytes, macrophages/microglia and fibroblasts needs to be further investigated.
To directly explore the effect of the PDGFRβ pathway, a single factor, on fibrotic scar formation, we injected exogenous PDGFB or PDGFD into the uninjured spinal cord instead of the injured spinal cord to avoid the influence of the complex microenvironment of SCI. Our results showed that both PDGFB and PDGFD can promote fibrosis in the uninjured spinal cord, and the profibrotic effect can be blocked by the PDGFRβ inhibitor SU16f. The results of FN- or LN-labelled fibrosis was consistent with those of PDGFRβ-labelled fibrosis. Therefore, our results were reliable and preliminarily confirmed that the activation of the PDGFRβ pathway is sufficient to induce fibrosis. Notably, SU16f completely blocked PDGFD-induced fibrosis but only partially blocked PDGFB-induced fibrosis in the uninjured spinal cord, suggesting that PDGFB and PDGFD may be involved in different phases of fibrotic scar formation. We emphasize that the process and mechanism are worthy of in-depth study. In addition, SU16f blockade of the PDGFRβ pathway was performed to further confirm the effect of the PDGFRβ pathway on fibrotic scar formation after SCI. The results showed that SU16f significantly inhibited the proliferation of fibroblasts and reduced fibrotic scar after SCI. Therefore, our results provide direct evidence that the PDGFRβ pathway mediates fibrotic scar formation after SCI, which can be blocked by SU16f inhibiting the proliferation of fibroblasts.
The dense contiguous fibrotic/astrocytic scar boundary is an important component of the inhibitory microenvironment after SCI [
1]. The physical barrier of the scars directly prevents the regenerated axons from passing through the injured core, and the axon tips form retraction bulbs after contacting fibrotic scar, resulting in the failure of axon regeneration after SCI [
6]. Therefore, the interruption of the contiguous scar boundary contributes to axon regeneration [
6]. In our study, the results showed that SU16f blockade of the PDGFRβ pathway resulted in the interruption of the fibrotic/astrocytic scar boundary and the reduction of the lesion size after SCI, facilitating the regeneration of NF
+ or 5-HT
+ axons that passed through the injured core. Interestingly, our results showed that SU16f-induced reduction in fibrotic scar led to a smaller area of inflammatory cells at 28 dpi. The Jonas Frisén group used Glast–Rasless transgenic mice to completely eliminate fibrotic scar after SCI, leading to the spread of inflammatory cells at 14 dpi. However, a moderate reduction in fibrotic scar did not lead to the spread of inflammatory cells at 14 dpi but led to a reduction in inflammatory cells at 28 dpi [
4,
6]. Therefore, our results are consistent with the results of the Jonas Frisén group, together indicating that moderate inhibition of fibrotic scar after SCI does not lead to the spread of inflammation in the early stage but inhibits the spread of inflammation in the late stage, which contributes to axon regeneration. Blood-derived macrophages migrate towards high concentrations of complement component C5a in the injured core after SCI, and C5a may be secreted by PDGFRβ
+ fibroblasts [
43], suggesting that C5a may be involved in fibroblasts corralling macrophages in the injured core. The effect of fibrotic scar changes on inflammatory response after SCI and its molecular mechanism need to be further investigated. Overall, our results further reveal that the adverse effects of excessively deposited fibrotic scar are greater than its beneficial effects in SCI and can be used as a therapeutic target after SCI. Finally, the results of BMS score and footprint analysis confirmed that SU16f blockade of the PDGFRβ pathway promotes locomotor function recovery in injured mice. Although fibrotic scar forms after SCI in both rats and mice [
7], SCI in rats leads to cavity formation in the injured core, which is considered to resemble the pathological changes in patients with SCI in the clinic [
44,
45]. Our findings should be further validated in rat models, which could contribute to a better understanding of fibrotic scar as a therapeutic target for SCI in clinic. Thus far, specific therapeutic targets for inhibiting fibrotic scar formation after SCI have rarely been reported, and the present study is expected to provide a novel idea.
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