Background
Spinal cord injury (SCI) typically causes irreversible motor and sensory deficits that result in enormous socioeconomic burden [
1,
2]. There are approximately 2.5 million people globally who live with SCI, and this figure is thought to grow by 130,000 annually [
3]. No pharmacological therapy that can effectively treat this condition exists [
4]. The pathogenesis of SCI starts with mechanically inflicted trauma that incites primary and secondary injury phases. The secondary injury phase in SCI has been proven to function as a significant therapeutic window, during which neuroprotective treatment may be implemented in efforts to bolster functional recovery after SCI [
5,
6]. Existing evidence implicates inflammation in the pathogenesis of secondary injury after SCI [
6‐
8].
Macrophages are vital mediators of inflammation in central nervous system (CNS) injury. Macrophages have been found to either be of peripheral myeloid origin that infiltrates the CNS in response to injury or may originate from the CNS itself from the resident microglial population [
9]. Microglia develop from embryonic yolk sac cells that migrate to the spinal cord at 11.5 days post-fertilization. Macrophages/microglia are extremely adaptable cells and are able to transform to functionally different phenotypes in response to changes in their microenvironment [
10]. They are thought to exist as two polarized forms that confer opposite effects on the prognosis of CNS disorders—the M1 phenotype has been shown to be pro-inflammatory and cytotoxic, while the M2 phenotype demonstrates pro-repair and anti-inflammatory functions [
9].
Cells regulate their immune and inflammatory responses via the binding of extracellular adenosine to G-protein-coupled adenosine receptors found on the surface of effector cells [
11]. This is a critical process, and increasing volumes of literature highlights the neuroprotective role of nucleoside adenosine in injured brain and spinal cord tissues [
12]. This beneficial phenomenon is thought to be due to the involvement of adenosine in the activation of macrophages and microglia [
13,
14].
Ecto-5′-nucleotidase (CD73) is a 70 kDa glycosylated protein that exists on the external plasma membrane layer and functions to hydrolyze extracellular AMP into adenosine and phosphate [
15]. About 85–95% of murine AMP-hydrolyzing capabilities are mediated by CD73, which is a major murine cerebral 5′-nucleotidase [
16]. By modulating extracellular adenosine levels, CD73 is thought to be intricately associated to immune- and inflammatory-related cerebral developmental diseases [
17,
18]. However, its effect on secondary spinal cord injury remains unknown.
The current study hypothesized that altered CD73 expression could therefore affect macrophages/microglia polarization through adenosine. Our experiments were two-pronged and first aimed to characterize CD73 expression in spinal cord after SCI and secondly to further delineate its role in the macrophages/microglia polarization both in vitro and in vivo.
Methods
Animals
C57BL/6 CD73 knock out (KO) male mice were kindly gifted by Prof. Thompson, Oklahoma Medical Research Foundation, Oklahoma City, USA. The Shanghai SLAC Laboratory Animal Co., Ltd. (Shanghai, China) supplied the wild-type (WT) male C57BL/6 mice.
Surgery
Each mouse was inflicted with spinal crush injury at the midthoracic region (T8–T9) with Dumont-type forceps with a 0.2 mm spacer, as described previously [
19]. Firstly, T8-T9 vertebrae laminectomies were carried out with a pair of fine rongeurs, with precautions taken to preserve the dura. Lateral compression of the spinal cord was applied to achieve a depth of 0.2 mm for 20 s. Mice were reared separately post-surgery and administered twice-daily manual bladder expression. Control mice received sham surgery with laminectomy but without damage to the spinal cord.
Quantitative real-time PCR (qPCR) analysis
Total RNA extraction was performed with TRIzol reagent (Invitrogen, San Diego, CA, USA) based on the protocol supplied by the manufacturer, and the SYBR Green reagent was used in qPCR mRNA quantification. The housekeeping gene used was GAPDH, and the comparative ΔΔCT method was used to calculate mRNA relative expression levels.
Assessment of motor function
Overall locomotor function was recorded utilizing the Basso, Beattie, and Bresnahan (BBB) locomotor recovery scale. It is a 21-item scale that is based on observations in stepping, coordination, and hindlimb movement in the open-field test, with 0 indicating no spontaneous locomotor activity and 21 indicating normal coordinated gait with parallel paw placement. Four groups (WT+Sham, WT+SCI, KO+Sham, KO+SCI) were assessed on post-operative days 1, 3, 7, 14, 21, 28, 35, and 60. Each mouse was observed for 4 min in the open-field test by an assessor blinded to the treatments.
Histological and immunohistochemical assessment
Sham and SCI mice (n = 4) were deeply anesthetized with 10% chloral hydras (3.5 ml/kg, i.p.) 3 days after surgery. 0.9% NaCl was then used to perfuse the mice, followed by 4% paraformaldehyde in 0.01 M phosphate-buffered saline (PBS, pH = 7.4). Spinal cord tissue at the region of injury were dissected with a 0.5 margin on each side of the lesion and embedded in paraffin. Hematoxylin-eosin (HE) staining was then used for histopathological assessment on 25-μm-thick transverse paraffin sections mounted on poly-L-lysine-coated slides. The sections were also subjected to Nissl staining by incubation in 1% cresyl violet acetate and were examined under a light microscope. For immunohistochemical analysis of TNF-α, IL-1β, and CD73, paraffin was first removed from the sections, endogenous peroxidase blocked by a 10-min H2O2 incubation and 10 min methanol incubation, and finally for 30 min in serum-blocking solution. Sections were then incubated with TNF-α (1:100, Abcam, ab6671), IL-1β (1:100, Abcam, ab9722), and CD73 (1:100, Abcam, ab175396) antibodies for 1 h, followed by incubation with HRP-conjugated anti-rabbit secondary antibodies for 30 min. DAB was then added to the sections and incubated for 10 min to allow visualization of brain segments containing bound antibodies. All incubation processes were carried out at room temperature. A Nikon ECLIPSE Ti microscope (Nikon, Japan) was used for imaging. Semi-quantification of integrated optical density (IOD) and area was done with the help of Image Pro Plus 6.0.
Immunofluorescence assessment
As described previously, samples of spinal cord tissue were extracted at day 3 post-surgery. After transfection for 24 h and LPS/IL-4 induction for 8 h, BV2 cell samples were fixed with 4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4) for 15 min. All samples were then immersed for 1 h with 1% bovine serum albumin and 0.3% Triton X-100 in order to block all reactions. This was followed by an overnight incubation at 4 °C with the following primary antibodies: CD68 (1:100; Abcam, ab201845), CD73 (1:100, Abcam, ab175396), iNOS (1:100; Abcam, ab49999), and Arg1 (1:100, Abcam, ab133543). The next day, all samples underwent PBS washing and 2 h room temperature incubation with their corresponding secondary antibody: Dylight (Dy)488- and Dy594-conjugated secondary antibodies (all 1:1000; Jackson ImmunoResearch, West Grove, PA). Imaging was performed with either the Nikon ECLIPSE Ti microscope (Nikon, Japan) or Olympus FV 1000 confocal microscope (Olympus, Tokyo, Japan).
Inflammatory cytokine array
The conditioned media containing transfected and IL-4 treated BV2 cells as well as lysate from injured spinal tissue were concentrated and analyzed with the Ray Biotech (Norcross, GA) Mouse Inflammation Antibody Array G series I and processed in accordance with the manufacturer’s protocol.
Western blot analysis
A RIPA buffer (25 mM Tris•HCl pH 7.6, 150 mM NaCl, 1% NP-40, 1% sodium deoxycholate, 0.1% SDS) was used to extract total protein for subsequent concentration analysis with the BCA assay. SDS-polyacrylamide gel electrophoresis was used to dissolve protein samples, which were then transferred to nitrocellulose membranes. Subsequently, a 1-h incubation with 5% skim milk in TBST (25 mM Tris, 150 mM NaCl, 0.05% Tween-20, pH 7.5) was used to block reactions. Membranes were then incubated with antibodies (1:1000 dilutions for all antibodies) overnight at 4 °C. A HRP-conjugated secondary antibody was then added to the sections for a 1-h incubation at ambient temperature, and section colors were subsequently developed with ECL. A gel imaging system (UVP LLC, Upland, CA, USA) was used to image results which were then measured using Gel-Pro Analyzer software (Media Cybernetics, Rockville, MD, USA).
Cell cultures
BV2 cells were grown in DMEM (Gibco, Carlsbad, CA, USA) supplemented with 10% FBS (Gibco, Carlsbad, CA, USA), 50 g/ml streptomycin (Invitrogen, Carlsbad, CA, USA), and 50 U/ml penicillin in a humidified atmosphere of 95% air and 5% CO2.
Construction and transfection of siRNA and pcDNA3.1 plasmids
Plasmid construction was guided by previous reports [
20,
21]. Briefly, three CD73 DNA sequences (CGTTGGATACACTTCCAAA, GGAGGACACTCCAACACAT, and CAACGTGGTTTCTACATAT) were selected for designing the siRNA target. Based on the U6 siRNA expression vector, pRNAT-U6.1/Neo vector (GenScript Corp., Piscataway, NJ, USA), three CD73 siRNA plasmids were built. In addition, the CD73 gene was extracted from the pBluescript SK(±) vector and cloned into the unique BamHI and KpnI pcDNA3.1 expression vector cloning sites. All constructs were confirmed by sequencing. The plasmid siRNA-CD73 and pcDNA3.1-CD73 were transfected into BV2 cells using Lipofectamine™ 2000 reagent (Invitrogen, Carlsbad CA, USA) following the protocol stipulated by the manufacturer.
Statistical analysis
All results are expressed as mean ± standard deviation. Student’s unpaired t tests and two-way analysis of variance (ANOVA) followed by Dunnett’s test were used to analyze data. A p value of less than 0.05 was considered to be statistically significant. All statistical analyses were done with the SPSS 14.0 software.
Discussion
Trauma inflicted on the CNS, either by SCI or traumatic brain injury (TBI), often results in widespread inflammation, which leads to marked neuropathology and limited functional recovery [
6,
29]. Researchers have confirmed the accumulation of pro-inflammation cytokines that encompasses TNFα, IL-1β, and IL-6 are able to activate microglia and astrocytes, resulting in a secondary cytotoxic response involving free-radical and vasoactive amines, eventually culminating in neuronal apoptosis [
30,
31]. The current study revealed an upregulation of pro-inflammation cytokines in spinal cord tissue that peaked at 3 days post-SCI, and Caspase 3 mRNA expression indicates that significant neuronal apoptosis may be taking place, especially on the third day after SCI (Additional file
1). Based on these results, we can safely conclude that neuroinflammation plays a crucial role in the secondary injury phase of SCI.
CD73, a glycosylphosphatidylinositol (GPI) anchored cell surface protein, has a central role in adenosine signaling, working to catalyze AMP into phosphate and adenosine. This process has been linked to the proliferation, migration, invasion, and drug resistance of various cancer cells [
20,
32,
33]. However, literature is scarce with regard to the effects of CD73 in SCI. Baud et al. reported that CD73 is abundant in neurons and gliocytes [
34]. In 1997, Braun et al. described that focal cerebral ischemia enhanced glial expression of CD73 [
22]. Additionally, Petrovic-Djergovic et al. demonstrated that CD73 was able to reduce infarcted area in an ischemic brain by regulating leukocyte trafficking [
35]. In the bilateral common carotid artery stenosis model of cerebral hypoperfusion, Hou et al. demonstrated significantly elevated pro-inflammation cytokine levels in the presence of CD73 deficiency [
17]. Together, these studies provide important insights into the neuroprotective effect of CD73 on the CNS. In the current study, SCI induced microglia CD73 expression (Figs.
1 and
2). CD73-deficient mice demonstrated worse motor dysfunction, more potent immune responses, as well as increased tissue destruction and cell apoptosis (Fig.
3). In the current study, no changes in BBB score were found at 3 days post-injury, the time point at which CD73 were upregulated. This discrepancy could be attributed to the plasticity of motor system after SCI. The recovery process after SCI can go on for several years and probably depends on the reorganization of circuits that have been spared by the lesion [
36]. Within the first days after SCI, the initial recovery of function is mostly due to metabolic changes at the site of damage [
37]. Synaptic plasticity in pre-existing pathways and the formation of new circuits through collateral sprouting of lesioned and unlesioned fibers occur subsequently [
38]. So at 3 days post-injury, it seems possible that no changes in the BBB score between CD73 knocked out mice and wild-type mice because the synaptic plasticity and formation of new circuits have not even started. We considered that this also may be the reason why so many researches demonstrated different SCI therapies worked until 2 weeks after injury evaluated by BBB score [
39‐
41]. In the research of Hsu et al., they demonstrated MMP-2 was neuroprotective after SCI, and the locomotor scores show a significant difference between MMP-2 deficient mice and wild-type mice until 42 days post-injury [
42]. We also tried micro-MRI to evaluate the spinal trauma, and the T2 images showed that CD73-deficient mice had more extensive marrow edema and enhancement at 3 days post-injury (Additional file
2). These results are congruent with those obtained in earlier studies.
As a crucial CNS immune cell, macrophages/microglia are the first cells to be recruited in response to tissue injury or infection. They are able to manifest in two states with opposing functional characteristics, known as M1 (the classical pro-inflammatory macrophages/microglia) and M2 (the alternatively activated anti-inflammatory macrophages/microglia) [
43‐
45]. Kigerl et al. observed that IL-4 polarized M2-conditioned media stimulated axonal proliferation in cultured neurons [
46]. David et al. identified the association between alternative activation of microglia and improved neurological outcome after SCI in mice [
47]. Moreover, Anhui et al. showed that inflammation after SCI was inhibited by programmed cell death-1 (PD-1) promoted M2 polarization [
45]. When interpreted as a whole, these papers demonstrate that microglia/macrophages M2 activation can mitigate spinal cord damage associated with SCI.
Interestingly, our research also revealed that CD73 had the ability to disrupt macrophages/microglia polarization. The expression of pro-inflammatory activation markers in CD73-KO mice was significantly elevated while anti-inflammatory activation marker expression was suppressed at both the mRNA and protein levels, indicating the CD73 had a protective effect on secondary injury in SCI by promoting microglia/macrophages expressed Arg1, IL-10, and other anti-inflammation cytokines (Fig.
4). The in vivo results are also reflected in the in vitro models in this study. There is a marked accumulation of proteins characteristic of M1 microglia cells in CD73-KO BV2 cells that were treated with LPS, while the enhancement of M2 microglial markers was more significant when the CD73-KO BV2 cells were treated with IL-4 (Figs.
5 and
6). Moreover, the inflammatory cytokine array detected relatively low expressions of IL-10 and TGF-β in CD73-deficient cells and injured spinal cord tissue (Fig.
7). It is therefore likely that CD73 is intricately related to microglia/macrophages activation. Our findings are in contrast to those of Eichin et al., who found that CD73 is not a necessary factor for the polarization of M2 macrophages [
48]. A possible explanation for this might be the differing experimental microenvironments resulting in different CD73 responses in microglia and macrophages.
Extracellular adenosine stimulates a myriad of protective cellular responses that has the ability to restore homeostasis [
11]. There is experimental evidence that adenosine confers a disruptive quality to the pathogenic processes that are triggered by acute traumatic injuries, hinting towards the potential neuroprotective properties of adenosine receptor activation [
49‐
52]. The effects of adenosine are triggered when it binds to and activates between one and four G-protein-coupled transmembrane adenosine receptors (ARs), designated A
1, A
2A, A
2B, and A
3 [
53,
54]. Prior studies have observed that LPS-induced production of IL-12 and TNF-α by microglia is mitigated when adenosine is allowed to interact with A
2A and A
3 [
55,
56]. Csoka et al. found that adenosine promoted alternative macrophage activation [
13], Koscso et al. discovered adenosine augmented microglial IL-10 production, and both studies indicated that adenosine’s protective effect was dependent on activation of the adenosine A
2B adenosine receptor [
14]. Hence, it could conceivably be hypothesized that CD73 promotes microglia/macrophages M2 polarization by adenosine signaling. In this study, using nonselective agonist and A
2B selective antagonist, we demonstrated that the A
2B receptor is primarily responsible for stimulating CD73 to trigger microglia alternative activation (Fig.
8).
External stress signals are conveyed to cellular nuclei via mitogen-activated protein kinases (MAPKs). MAPK signaling can be stimulated by A
2A and A
2B receptors [
57], while the p38-activating ability of adenosine has been demonstrated in macrophages and microglia [
58‐
60]. Furthermore, the participation of p38 in the alternative activation of macrophages via A
2A and A
2B receptors also has been reported [
13,
14]. These results provide support for the hypothesis that p38 has a crucial role in allowing CD73 to modulate microglial M2 polarization. We found that although IL-4 or CD73 overexpression alone had no effect on p38 phosphorylation, a combination of both increased p38 activation, and this effect can be mimicked or counteracted by NECA or MRS1706 (Fig.
9a–d). Additionally, p38 pathway inhibition negated the stimulating effects of CD73 on microglial M2 polarization induced by IL-4 exposure (Fig.
9e–j).