Background
Parkinson’s disease (PD) is the second most common neurodegenerative disorder characterized by a massive and preferential loss of dopaminergic (DA) neurons in the substantia nigra (SN) and a drastic decline in striatal dopamine concentrations. It is established that sustained neuroinflammation has been suggested to contribute to the pathogenesis of PD [
1,
2]. Activated microglia and astrocytes, together with elevated levels of inflammatory mediators and cytotoxic factors including tumor necrosis factor alpha (TNFα); interleukin (IL)-1β, IL-2, and IL-6; inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX2), have all been observed in the brain of PD patients [
3‐
6], as well as in the 6-hydroxydopamine (6-OHDA) [
7‐
10], 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) [
11,
12] and rotenone animal models of PD [
13]. However, the mechanisms underlying neuroinflammation currently remain unclear.
Accumulating evidence suggests that activation of innate immunity via Toll-like receptors (TLRs) [
13‐
19] and nucleotide-binding oligomerization domain-containing protein (NOD)-like receptors (NLRs) [
20,
21] has been implied to participate in the pathogenesis of PD. NOD2, one of the first discovered NLRs, plays an important role in regulating inflammatory homeostasis. It recognizes not only the exogenous pathogen-associated molecular patterns (PAMPs) such as muramyl dipeptide (MDP), a degradation product of peptidoglycans from virtually all bacteria [
22], but also endogenous danger-associated molecular patterns (DAMPs) which are released from the damaged tissues following cellular stress. When engaged by its ligands, NOD2 recruits kinase receptor-interacting serine/threonine-protein kinase 2 (RIP2), leading to the activation of NF-κB and MAPK signaling pathways [
23]. It has been reported that mutations in the NOD2 gene are related to inflammatory diseases such as Crohn’s disease, Blau syndrome, and early-onset sarcoidosis [
24‐
27]. However, the data on such mutations in PD are inconclusive [
28‐
30]. This study was designed to elucidate the function of NOD2 signaling and its regulatory mechanism in the pathogenesis of PD.
Methods
Animals
Ten-week-old male C57BL/6J mice (weight 23–28 g) were obtained from the Experimental Animal Center of Shandong University. NOD2 knockout (NOD2−/−) mice on a C57BL/6 background were purchased from the Jackson Laboratory (Bar Harbor, ME, USA). All mice were housed under standard conditions of temperature and humidity, with a 12-h light/dark cycle and free access to food and water. All animal experiments were pre-approved by the Institutional Animal Care and Use Committee of Shandong University.
Striatal injections
Mice were placed in a stereotaxic device under 1.5% pentobarbital sodium anesthesia and given 2 μl of 3 μg/μl 6-OHDA (Sigma-Aldrich, H4381) which is dissolved in sterile normal saline (NS) with 0.02% ascorbic acid or 4 μg/μl MDP (an agonist of NOD2, Sigma-Aldrich, A9519) solution into two different sites of the right striatum (STR) separately: point A—1.0 mm anterior and 2.1 mm lateral to the bregma and 2.9 mm from the dura mater; point B—0.3 mm posterior and 2.3 mm lateral to the bregma and 2.9 mm from the dura mater. The injection was conducted at a rate of 0.5 μl/min, and the needle was left in place for an additional 4 min before it was slowly removed. The control group was injected with NS alone (containing 0.2% ascorbic acid) into the STR.
Apocynin treatment
To evaluate the role of NADPH oxidase in NOD2 signaling in PD, mice were given saline or apocynin (15 mg/kg), a NADPH oxidase inhibitor, by daily intraperitoneal injection for 14 days after 6-OHDA and MDP administration.
Apomorphine-induced rotation test
Rotation testing was performed according to a previously published protocol [
31]. One day, 2 days, 3 days, 7 days, 14 days, and 21 days after 6-OHDA injection, the mice were injected subcutaneously with 0.1 mg/kg apomorphine hydrochloride (Sigma-Aldrich, A4393). Then, the mice were placed individually in a transparent plastic cylinder (diameter 13 cm) and allowed to adapt to their environment for 5 min before the rotations were recorded. The total number of full 360° rotations in the contralateral direction was counted for 30 min. Results were expressed as average rotations per 10 min in 30-min measurement.
RNA extraction and RT-PCR
RNA extraction and reverse transcription were performed as described previously [
32]. The quantitative real-time PCR analysis was performed with SYBR green (Invitrogen) using a Bio-Rad iCycler system (Bio-Rad). Housekeeping gene expression of β-actin was used for normalization. Relative expression was determined by the 2
−ΔΔCT method. The specific primers used were β-actin (sense 5′-GAATTGCTATGTGTCTGGGT-3′, antisense 5′-CATCTTCAAACCTCCATGATG-3′) and NOD2 (sense5′-GCCTTCCTTCTACAGCACGT-3′, antisense 5′-TGGCAGGGCTCTTCTGCAAG-3′).
Immunoblotting
Proteins were prepared as previously described [
33]. The membranes were incubated with the following primary antibodies: TH (Millipore, MAB318, 1:2000), NOD2 (Proteintech, 20980, 1:1000; Group), NOX2 (BD, 611414, 1:1000), Bax (Proteintech, 50599, 1:2000), Bcl-2 (Proteintech, 12789, 1:1000), cytochrome C (Proteintech, 10993, 1:1000), cleaved caspase-3 (Bioworld, BS7004, 1:500), pro caspase-3(Bioworld, BS9865, 1:1000), COX2 (Proteintech, 12375, 1:500), iNOS (Proteintech, 18985, 1:500), NF-κB p-p65 (Cell signaling, 3033, 1:500), NF-κB p65 (Proteintech, 10745, 1:1000), IκBα (Bioworld, BS3601, 1:1000), or β-Actin (Bioworld, AP0060, 1:5000). The membranes were then incubated with HRP-conjugated secondary antibodies for 1 h. Subsequent visualization was performed using an enhanced chemiluminescence system (ECL, Millipore). Densitometry of Western blot bands was assessed with the image lab program and then normalized to the intensity of β-actin.
Immunohistochemistry
Mice were anesthetized with sodium pentobarbital 14 days after 6-OHDA or MDP administration, and the brains were perfusion-fixed with 4% paraformaldehyde (PFA) following 0.9% NS. Then, the brains were post-fixed in 4% PFA for 24 h and cryopreserved in 30% sucrose in turns. The brain sections (20 μm or 10 μm) were obtained on a sliding microtome adapted for cryosectioning. The sections were incubated with the following primary antibodies: NOD2 (Proteintech, 20980, 1:200), TH (Millipore, MAB318, 1:200), Iba1 (Proteintech, 10904, 1:200), glial fibrillary acidic protein (GFAP) (Millipore, MAB360, 1:200), and Neun (Millipore, MAB377, 1:200). For immunofluorescence staining, the sections were counterstained with DAPI. The morphometric analyses were performed using the program Image J (NIH, Bethesda, MA).
Dopamine and its metabolites, dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were measured using HPLC/MS method. Briefly, the right striatum was individually weighed and homogenized in ice-cold 0.5 M formic acid with the concentration of 5 ml/g tissue. Lysates were centrifuged at 15,000
g for 30 min at 4 °C. The supernatant was separated and analyzed according to the established protocols with minor modifications [
34]. The concentration was expressed as nanogram per milligram tissue.
TUNEL staining
TUNEL staining was performed using an in situ apoptosis detection kit (Roche, 11684817910) according to the manufacturer’s instructions. TUNEL-positive cells displayed brown staining within the nucleus, and the number of TUNEL-positive cells was counted in three non-overlapping microscopic eyeshots by a person blinded to the group assignment under high-power magnification (× 200) and displayed as a percentage.
Cytometric bead array assay
IL-6, IL-12p70, MCP-1, TNFα, and IL-10 in mice SN were captured by cytometric bead array (BD, 552364) according to the manufacturer’s manual. Cytokine levels were then quantified by flow cytometry (Beckman Coulter).
ROS measurement
ROS generation in the SN or microglia was measured by the fluorescence intensity of dichlorofluorescein (DCF) converted from 2′,7′-dichlorofluorescein diacetate (DCFH-DA) at 525 nm after excitation at 488 nm by a fluorescence plate reader (Thermo Scientific Varioskan Flash).
Cell culture and treatment
Microglia BV2 cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM; Gibco) supplemented with 10% fetal bovine serum (FBS; Gibco). The neurotoxin 6-OHDA was dissolved in 0.02% ascorbic acid and prepared fresh for each experiment. Cultures were exposed to 50 μM 6-OHDA for indicated time before being harvested for various assays. shRNA-NOX2 was synthesized by GenePharma Co., Ltd. (Shanghai, China). The target sequence for shRNA-NOX2 (5′-GAGTGGTGTGTGAATGCCAGA-3′) was designed based on the core sequence of mouse NOX2 cDNA (accession number: NM_007807.4). Transfection was performed using lipofectamine 3000 reagent (Invitrogen).
The human neuroblastoma cell line SH-SY5Y cells were maintained in DMEM-F12 (Hyclone) containing 10% FBS. Conditioned medium (CM) from 6-OHDA or MDP-treated BV2 cells was collected from wells, pooled, and centrifuged at 170g for 5 min to remove cell debris. SH-SY5Y cells were cultured in 96-well plates at 10,000 cells/well and were incubated for 24 h before the addition of BV2 CM. The original media was removed from SH-SY5Y cell cultures and replaced with 100 μl of DMEM-F12 mixed with 100 μl of BV2 CM. The SH-SY5Y cells were then incubated for 24 h. Cell viability was determined by the Cell Counting Kit-8 (CCK-8) (Beyotime, C0038) assay.
Statistical analysis
Data analyses were performed using the SPSS statistical software. Statistical significance between multiple groups was analyzed by one-way ANOVA followed by LSD post hoc test. When equal variances were not assumed, Dunnett’s multiple comparisons test was used to compare the differences between the groups. Comparison of the two groups was performed using two-tailed t test. All data are presented as mean ± SEM, and p < 0.05 was considered a statistically significant difference.
Discussion
Here, we reported that NOD2 played a critical role in 6-OHDA-induced PD-like pathogenesis. We found that NOD2 was upregulated in the SN and STR in PD model mice induced by 6-OHDA. The striatal injection of NOD2 agonist MDP induced DA degeneration mice, while mice lacking NOD2 protected against the DA degeneration induced by 6-OHDA or MDP due to the attenuated inflammatory response. We further demonstrated NOX2-mediated oxidative stress linked NOD2 to DA neuronal degeneration induced by 6-OHDA.
Neuroinflammation is an important contributor to the neuronal loss in PD [
2]. NOD2 has several important properties, which contribute to brain inflammation. It was identified as an important component in the generation of damaging central nervous system (CNS) inflammation following bacterial infection [
36]. We recently reported NOD2 was involved in the inflammatory response after cerebral ischemia-reperfusion injury [
33]. But the role and mechanism of NOD2 in PD are still unknown. Although there is no consensus on the genetic evidence supporting a pathogenic function of NOD2 in PD [
28‐
30], we proved the NOD2 expression in the SN and striatum was upregulated in 6-OHDA-induced PD model mice in the present study. WT mice with the treatment of NOD2 agonist MDP displayed the injury of DA neurons, while NOD2
−/− mice were partially protected against 6-OHDA toxicity. These findings indicated that DA cell death in this paradigm is, at least to some extent, NOD2-dependent.
Microglia, the resident innate immune cells, are sensitive to even minor disturbances in CNS homeostasis and become readily activated during most neuropathological conditions [
37]. Activated microglia is thought to promote neuronal damage, particularly in neurodegenerative diseases, via the release of pro-inflammatory and neurotoxic factors. Several studies have provided evidence that exposure of microglia to PAMPs or DAMPs leads to the release of inflammatory and toxic molecules that contribute to neurodegeneration [
38,
39]. The astrocytic response has also been reported to play an important role in the events leading to DA degeneration, a response which seemingly occurs following microgliosis [
19]. We observed the obvious activation of microglia and astrocytes displayed in WT mice treated with 6-OHDA or MDP, while this activation was significantly attenuated by NOD2 deficiency. It has been shown that activated microglia can produce and release, in excess, a host of harmful compounds such as ROS, reactive nitrogen species, and pro-inflammatory cytokines. Evidence has also suggested the activities of nuclear factor kappa B (NF-κB) are the potential mechanisms mediating activated microglia-associated DA degeneration [
40]. NF-κB activation drives the expression of inflammatory cytokines in PD. Our results also showed that the activation of NF-κB, as evidenced by IκBα degradation and p65 phosphorylation, was significantly inhibited in NOD2
−/− mice compared with WT mice treated with 6-OHDA or MDP. Congruently, inflammatory cytokines including MCP-1, IL-6, IL-12, and TNFα were produced in less amount in NOD2
−/− mice. These findings for the first time demonstrated that NOD2 is one of the critical components in a signal transduction pathway that links 6-OHDA toxicity to inflammation in PD.
Apoptosis plays a fundamental role in the regulation of cellular homeostasis and is involved in DA neuronal loss in PD [
41,
42]. NOD2 has been causally linked to the pathogenesis of apoptosis [
43]. In our study, treatment with 6-OHDA or MDP resulted in a significant increase in apoptosis of DA neurons in the SN from WT mice, while NOD2 deficiency reduced the apoptosis induced by 6-OHDA or MDP. Caspase-3, a member of the cysteine proteases, is implicated in the apoptosis. Activation of caspase-3 is associated with a series of signal transduction cascades including cytochrome C, Bcl-2, and Bax proteins. Bcl-2 can form a heterodimer with Bax, thereby preventing Bax homodimerization and causing an alteration in mitochondrial permeability. The change in the permeability of mitochondria leads to cytochrome C release, formation of the apoptosome, and the subsequent activation of caspase-3 [
44]. Furthermore, the activation of caspase-3-like proteases has been shown to be involved in the pro-apoptotic function of neurotoxins in MPTP and 6-OHDA models [
45]. MPTP-induced dopaminergic neurodegeneration was paralleled with the upregulation of Bax and downregulation of Bcl-2 in the SN, and mutant mice lacking Bax are more resistant to MPTP than their WT littermates [
46]. The present study revealed that the ratio of Bax/Bcl-2, cytochrome C, and caspase-3 activation was markedly inhibited in NOD2
−/− mice compared with WT mice when treated with 6-OHDA or MDP, suggesting the pro-apoptotic effect of NOD2 in PD. It is conceivable that NOD2 may exert its pro-apoptotic effects in PD by regulating cytochrome C, Bax, and Bcl-2, thus promoted the activation of caspase-3.
A particularly intriguing question is how NOD2 was upregulated in 6-OHDA induced PD model mice. Recent studies have indicated that activation of the immune system due to the disturbances in the redox state of cells seems to contribute to DA damage [
47], and NOX2-derived ROS are central to oxidative stress in PD [
48,
49]. Seven NOX isoforms have been identified, namely, NOX1, NOX2, NOX3, NOX4, NOX5, and dual oxidase 1 and 2 (DUOX1 and DUOX2). Structurally, all members of the NOX family contain a multisubunit structure, with catalytic flavin-binding NOX subunits and a number of regulatory subunits. Among them, NOX2 is the most important subtype for mediating PD injury. NOX2 is intensely expressed in microglial cells and has been demonstrated to be involved in the degeneration of DA neurons induced by MPTP or 6-OHDA [
48,
50,
51]. Our results showed NOD2 was also expressed in microglia and upregulated by 6-OHDA. Moreover, gene silencing of NOX2 suppressed the expression of NOD2 and the subsequent inflammatory response in microglia. We further confirmed that apocynin, a NADPH oxidase inhibitor, inhibited NOD2 upregulation and prevented DA degeneration in mice induced by 6-OHDA or MDP. These results suggested that NOX2-derived ROS contributed to the upregulation of NOD2 and the following inflammatory responses induced by 6-OHDA. In line with these findings, Shang et al. recently proved that enhanced NOD2 expression caused by high glucose in mesangial cells is regulated via the translocation of HuR, which is dependent on the NOX4-mediated redox signaling pathways [
52]. Of course, the detailed mechanisms of NOD2 regulated by NOX2 in PD needs to be further clarified. Moreover, we previously demonstrated NOX2-derived ROS mediated NOD2-dependent inflammatory responses in cerebral ischemia-reperfusion [
33]. Interestingly, we also found that the expression and activation of NOX2 induced by 6-OHDA or MDP were significantly inhibited in the NOD2
−/− mice compared to WT mice (Additional file
2: Figure S2). Overall, the results indicate that NOD2 and NOX2 form a positive circuit and promote DA degeneration in 6-OHDA-induced PD-like pathology in mouse models.