Inhibition of JNK Alleviates Chronic Hypoperfusion-Related Ischemia Induces Oxidative Stress and Brain Degeneration via Nrf2/HO-1 and NF-κB Signaling.

Cerebral ischemia is one of the leading causes of neurological disorders. The exact molecular mechanism related to chronic unilateral cerebral ischemia-induced neurodegeneration and memory deficit has not been precisely elucidated. In this study, we examined the effect of chronic ischemia on the induction of oxidative stress and c-Jun N-terminal kinase-associated detrimental effects and unveiled the inhibitory effect of specific JNK inhibitor (SP600125) on JNK-mediated brain degeneration in adult mice. Our behavioral, biochemical, and immunofluorescence studies revealed that chronic ischemic injuries sustained increased levels of oxidative stress-induced active JNK for a long time, whereas SP600125 significantly reduced the elevated level of active JNK and further regulated Nrf2/HO-1 and NF-κB signaling, which have been confirmed in vivo. Neuroinflammatory mediators and loss of neuronal cells was significantly reduced with the administration of SP600125. Ischemic brain injury caused synaptic dysfunction and memory impairment in mice. However, these were significantly improved with SP600125. On the whole, these findings suggest that elevated ROS-mediated JNK is a key mediator in chronic ischemic conditions and has a crucial role in neuroinflammation, neurodegeneration, and memory dysfunction. Our findings suggest that chronic oxidative stress associated JNK would be a potential target in time-dependent studies of chronic ischemic conditions induced brain degeneration.

1. Introduction

Ischemic stroke has long been reported as the most known cause of death globally [1, 2]. Cerebral ischemia alone is responsible for 80% of strokes that result from embolic or thrombotic blockade [3, 4]. It has clinical, social, and economic implications and requires significant efforts from both researchers and clinicians for understanding the underlying mechanisms [5]. There is emerging evidence that cerebral ischemia for a prolonged period may result in devastating effects via deteriorating ion gradient and by-products of anaerobic metabolism which disturbs brain homeostasis [6].

It has been reported that low but continuous blood supply to the brain results in dementia and slow neuronal disruption [7-9]. Several antioxidant enzymes play a critical role in maintaining a proper redox balance in brain cells [10, 11]. Among them, nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) are key redox-regulated proteins that play a major role against elevated ROS [12, 13]. Recently, a number of findings have reported that under normal conditions Nrf2 is localized in the cytoplasm. However, when oxidative stress increases, it translocates into the nucleus and binds with antioxidant response elements (ARE) and regulates many antioxidant encoding genes, particularly HO-1 [14, 15]. Oxidative stress induces activation of stress kinase such as c-Jun N-terminal kinase (JNK) that later mediates the deregulation of glutathione (GSH and GSSG) levels, which form an endogenous antioxidant system including Nrf2 and HO-1 in various neurodegenerative diseases. These deleterious conditions and failure of the antioxidant system, in turn, lead to the initiation of inflammatory cascades via translocation and activation of transcription factor NF-κB [16-18]. Several mechanisms have been studied which proposed that ischemia induces neuroinflammation via rapid activation of glial cells, the release of several proinflammatory cytokines, and by infiltration of different types of inflammatory mediators into the ischemic brain tissues [19].

Prior research shows that the c-Jun N-terminal kinase (JNK) plays an important role in cell proliferation, gene expression, and in cell apoptosis. Well-conducted studies provide documented evidence that JNK mediates various neuroinflammatory and neurodegenerative signals and their possible inhibition prevents neuroinflammatory responses and alleviated the synaptic dysfunction in a mouse model of cerebral ischemia [20, 21]. Based on this observation, we hypothesized that inhibition of active JNK with a specific inhibitor SP600125 could be a potential therapeutic target for ischemia induces brain degeneration and neuronal inflammation.

SP600125 (anthra [1, 9] pyrazol-6(2H)-one) is a well-known ATP-competitive JNK inhibitor that can cross the blood-brain barrier and could attenuate the expression of activated JNK in brain cells. SP600125 is a potent, cell permeable, selective, and reversible. Previous studies on numerous neurodegenerative diseases reported that the inhibition of active JNK via a specific JNK inhibitor SP600125 could abrogate neuroinflammation, neuronal apoptosis, and memory dysfunction [16, 22].

It is well-known that blood not only supplies oxygen but also provides essential nutrients and energy to the brain. Therefore, a deprivation of blood supply for 5–10 minutes may lead to irreversible brain injury [6, 23]. In 2014, Thong-asa and Tilokskulchai established a permanent unilateral right common carotid artery occlusion model in rats. They concluded that long-term right common carotid artery occlusion causes slow but progressive damage of dorsal hippocampal neurons [24]. Similarly, in the present study, we established a chronic unilateral cerebral ischemic mouse model through ligation and surgical cutting of the left common carotid artery. Herein, we investigated the effects of chronic ischemia-induced oxidative stress-mediated JNK activation on both the ipsilateral cortex and hippocampal regions and found that JNK is involved in the multiple pathological features of chronic cerebral ischemia. Furthermore, we confirmed that increased oxidative stress and active p-JNK might be involved in deregulating Nrf2/HO-1 signaling and the endogenous antioxidant system. Further, the activation of NF-κB leads to the initiation of neuroinflammation and neuronal apoptosis. Overall, our findings suggest that the inhibition of active JNK by a specific JNK inhibitor SP600125 could modify the chronic ischemia-associated neuropathology by regulating the Nrf2/HO-1 signaling and neuroinflammation.

2. Materials and Methods

2.1. Animals Used and Design of Their Groups

For the experiment, we purchase 10 weeks old (25–30 g), Male C57BL/6 N mice from Samtako Bio Labs, Olsan South Korea, and housed under controlled temperature (21 ± 2°C), relative humidity (60 ± 5%), and an artificial 12 h light/dark cycle, avoiding all stressful stimuli. The mice were kept in the animal care center of Gyeongsang National University, South Korea. All mice were acclimatized for seven days in the university animal house. After acclimatization of 1 week, mice were randomly divided (n = 15) into control vehicle-treated, ischemia alone, and ischemia+SP600125 treated groups. The grouping of the animals, and assessment of outcome, was based on blind bases. The experimenters were not blinded to the current study. The SP600125 treatment was started on the 22nd day of common carotid artery (CCA) ligation at a dose rate of 20 mg/kg/i.p/daily and continued up to the 28th day. The total SP600125 treatment duration was 7 days (Figures 1(a) and 1(b)). Animals were handled and processed according to the animal ethics committee (IACUC) of the division of applied life Sciences, Gyeongsang National University, Jinju South Korea (Approval ID: 125).

Figure 1

(a) Indicating mice grouping i.e. (1.) Control (2.) Ischemic (3.) Ischemia+SP600125. (b) Showing study plan for the current research work.

2.2. Anesthetics

For anesthetic purposes, Rompun (Xylazine) at a dose of 0.05 ml/100 g and Zolitil (Ketamine) at a dose of 0.1 ml/100 g of body weight were intraperitoneally (IP) administered to the mice.

After anesthesia, a straight incision was made into the neck region under hygienic conditions. After incision, the internal tissues and muscles were removed with blunt forceps, in order to prevent extra bleeding and capillary damage. Rectal temperature was maintained at 37°C ± 0.5°C during surgery up to the recovery from anesthesia using a self-regulating heating pad. The vagus nerve was isolated very gently, and the left common carotid artery was exposed and ligated with nonabsorbable suture material in head-tail direction and then cut with scissors in between the center. After suturing, the povidone-iodine was applied on the incision site to prevent infection and contamination. After surgery, normal saline was injected in order to prevent dehydration.

2.3. Behavior Study

2.3.1. Morris Water Maze (MWM) and Y-Maze Task

In order to familiarize the mice with the behavioral apparatus, we started the behavior study 18 days postsurgery. The MWM apparatus consists of a water tank 100 cm in diameter and 40 cm in height. To a depth of 15.5 cm, the tank was filled with water and the temperature was maintained at 25°C. The milk-like color of the water was made with white ink. A 10 cm platform, having 14.5 cm height was kept 1 cm below the water surface in one quadrant of the tank. On day 19th of the CCA ligation, the mice were trained regularly for 3 days for two hours on regular bases, mostly from 7 A.M. to 9 A.M. After completion of the training, the mice were adjusted for 24 hours, after that the experimental session was started from the 22nd day of the surgical procedures with SP600125 (20 mg/kg/IP/daily for 7 days) and continued for next five days. The time given for finding of the platform was kept at 60 s for each trial. On day 5, the probe test was performed. The hidden platform was removed, and mice were allowed to swim and find the platform point. The latency time to the platform, time spent on the target quadrant, and the number of crossings over the platform was calculated. After finishing the probe test, the Y-Maze test was performed. The Y-Maze is constructed of black wood, having a dimension of 50 cm length, 20 cm height, and 10 cm width. Each mouse was trained (1 hour) for the Y-Maze test. After 1 h, each mouse was placed in the center of the wooden apparatus and allowed to enter the apparatus arms without any hindrance. The series of arm entries was visually observed. Spontaneous alternation was defined as the successive entry of the mice into the three arms in overlapping triplet sets. Alternation behavior (%) was measured and calculated as (successive triplet sets divided by a total number of arm entries multiplied by 100). A video tracking system (SMART, Panlab Harvard Apparatus, Bioscience Company, USA) was used to record the movement of mice in the maze.

2.4. Protein Extraction from the Brain

For protein extraction, the mice were euthanized and the brains were removed. The left side of the hippocampus and cortex were dissected and homogenized in 0.2 M phosphate buffer saline (PBS) containing protease inhibitor cocktail followed by centrifugation. For further studies the proteins were stored at –80°C.

2.5. Western Blot Analysis

Western blot was performed as mentioned previously [25, 26]. In short, the proteins relative concentrations were analyzed using a Bio-Rad protein assay kit (Bio-Rad Laboratories, CA, USA) according to the instructions provided. Equal amounts of protein (20–30 μg) were electrophorized using 4–12% Bolt™ Mini Gels (Novex, Life Technologies Van Allen Way, Carlsbad, California). After running the proteins on Mini Gels, the membrane was kept in 5% (w/v) skim milk for 1 h in order to prevent nonspecific binding of the antibody. After this, the membranes were treated with the primary antibody overnight at 4°C. The next day, the membranes were treated with HRP- (horseradish peroxidase-) conjugated secondary antibodies. Washed with 1% TBST, the expression of the proteins was visualized with ECL (Amersham Pharmacia Biotech, Uppsala, Sweden) detecting reagents. The concentration of primary antibodies was 1 : 1000 dilution, and for the secondary antibodies, it was 1 : 10000 in 1% TBST. Beta-actin was used as a loading control. The results were obtained on X-rays films, which were scanned, and the optical densities were analyzed through densitometry using computer-based Image J software.

2.6. Immunofluorescence Staining

Both cortical and hippocampal regions were selected for immunofluorescence analysis as described previously [27, 28]. Briefly, the sections were washed for 10 min two times with phosphate buffer solution (PBS) and then treated with proteinase K (1 : 1000 dilution) for 5 min. Then, section slides were treated with a blocking solution (0.1% Triton X-100 and 2% normal goat/rabbit serum in 0.1 M PBS) for one hour at normal temperature. All the slides were incubated with the primary antibodies overnight at 4°C. Then, they were washed for 5 min with PBS followed by incubation (90 min at RT) with tetramethylrhodamine isothiocyanate (TRITC) and fluorescein isothiocyanate (FITC) labeled (1 : 100) secondary antibodies (Santa Cruz Biotechnology, Dallas, TX, USA). The slides were washed with PBS for 5 min 2 times and incubated with 4′, 6′-diamidino-2-phenylindole (DAPI) for 8–10 min and then covered with glass coverslips using fluorescent mounting medium. To analyze the antibody signals, a confocal laser-scanning microscope (FlouView FV 1000MPE, Olympus, Japan) was used.

2.7. Fluoro-Jade B (FJB) Staining

The staining was conducted as described previously [29, 30], with minor changes. The slides were washed with PBS for 10 min. Then, the slides were immersed in sodium hydroxide NaOH (1% W/V) + ethanol (80% v/v) for 5 minutes. The slides were dipped in ethanol (70% v/v) for 2 minutes followed by washing with distilled water for the next 2 minutes. Next, 60 mg KMNO4 was dissolved in 100 ml distal water (0.06% w/v), and all the slides were immersed in this KMNO4 solution for 10 min and then washed with distal water. The slides were added to the FJB solution (0.01% v/v) containing acetic acid (0.1%) for 15–20 mins. Then, all the slides were washed out with distal water 3 times for 1 min. Covered with cover slips, using a fluorescent mounting medium and the images were captured with a confocal laser scanning microscope (FluoView FV 1000MPE). The results were analyzed using the Image J program.

2.8. Nissl's Staining/Cresyl Violet Staining

The Nissl staining was conducted as mentioned previously with necessary changes [31]. In short, slides were dipped in PBS for 15 min then 0.5% Cresyl violet solution was applied for 10 minutes. Then, the slides were washed with distal water for 5 min and dehydrated in graded ethanol (70%, 95%, and 100%). The slides were allowed to dry in the hood (air Exeter), then immersed in xylene and cover slipped by using nonfluorescent mounting media. The results were examined using the computer-based Image J program.

2.9. ROS Assay In Vivo Samples

Reactive oxygen species generation was analyzed in vivo (brain tissues). The performance procedure was similar to that described previously [15, 32]. The assay was based on the conversion of 2′,7′-dichlorodihydrofluorescein diacetate (DCFH-DA) to 2′,7′-dichlorofluorescein (DCF).

2.10. LPO Assay In Vivo Samples

As previously described [33], the LPO levels were used to assess in vivo (brain tissue) through analyzing the malondialdehyde (MDA) level, a marker of LPO, by using the commercially available kit (catalog # K739-100) from BioVision Inc. (Milpitas, CA, USA). The assay was conducted as per the guidelines of the manufacturer.

2.11. Statistical Analysis

Sigma gel software (SPSS Inc., Chicago, IL, USA) was used to evaluate the scanned Western blots, whereas Image J software was used to analyze immunohistological findings. One-way ANOVA followed by Student's t-test was used to determine the mean ± SEM, whereas Graph Pad Prism 6 software was used for generating the graphs. Finally, P < 0.05 was taken statistically significant; ∗P < 0.05 represents significant differences between control and the ischemic group, whereas P < 0.05 showing a significant difference between ischemic and inhibitor.

3. Results

3.1. Chronic Unilateral Cerebral Ischemia Induces Oxidative Stress-Mediated JNK Phosphorylation, whereas SP600125 Reserves Their Expression

Brain is the prime organ that utilizes more oxygen than any other organ of the body and having more phospholipids; therefore, reactive oxygen species are continuously generated and produces oxidative stress. This oxidative stress later on activates JNK [34]. In initial experiments, we assessed the oxidative stress via ROS and LPO assay. Our results demonstrated that 28 days ischemia could induce oxidative stress (Figures 2(c) and 2(d)) which subsequently activate JNK.

Figure 2

Chronic ischemia induced the stress kinase p-JNK activation and deregulates Nrf2/HO-1 proteins. (a, b) Western blot findings of p-JNK in the different samples of the experimental groups. (c, d) Showing ROS and LPO assay results. (e, f) Immunofluorescent images representing the immunoreactivity of p-JNK (FITC; green color, DAPI; blue color) both in the cortex and hippocampal regions. (g, h) Western blot results of Nrf2/HO1, in the cortex, and hippocampus of the experimental groups. (i, j) Immunofluorescence results of Nrf2/HO1 in the experimental groups. In confocal microscopy the Magnification were kept 10x, Scale bar = 50 μm for all antibodies. In western blot assay, the β-actin was used as a loading control. Density values are relative to the control group (Rounding Rules) and expressed in arbitrary units (A.U). For western blot. The data are presented as the mean ± SEM. ∗Significantly different from the control group; #Significantly different from the ischemia group. Significance =  ∗P < 0.05, #P < 0.05.

The JNK is a stress kinase that activates and phosphorylates when oxidative stress rises in neuronal cells [35]. In this study, we examined the phosphorylation of JNK in the ipsilateral side of the cortex and hippocampus through Western blot. We found that chronic ischemia induces the phosphorylation of p-JNK both in the ipsilateral cortex and hippocampus of adult mice. On the other hand, we tested the effect of SP600125 on JNK inhibition and found that the administration of SP600125 at a dose rate of 20 mg/kg/IP/day for 7 days significantly reduced the expression level of p-JNK, showing its possible reversal effects. Similarly, our immunofluorescent results also supported the Western blot results that the ischemia group significantly increased phosphorylation of p-JNK, whereas SP600125 markedly reduced its immunoreactivity (Figures 2(a), 2(b), 2(e), and 2(f)).

3.2. Chronic Ischemic Condition Deregulates Endogenous Antioxidant Nrf2/HO-1 Pathway

Next, to assess the endogenous antioxidants expression in the chronic cerebral ischemia, we analyzed the master antioxidant regulators such as Nrf2 and HO-1 levels. Our Western blot results demonstrated that chronic cerebral ischemic conditions strongly affect the endogenous antioxidant system by reducing the expression of Nrf2/HO-1. To know the effect of activated JNK on these and to elucidate the downstream mechanisms of JNK on Nrf2/HO-1, we treated the SP600125, which induced therapeutic and potent antioxidant effects via increasing the expression of Nrf2/HO-1 protein level (Figures 2(g)–2(j)). These findings confirm that the inhibition of active p-JNK by SP600125 prevents the deregulation of its downstream Nrf2/HO-1 signaling.

3.3. Chronic Ischemia Induces Neuroinflammatory Cascades in Ipsilateral Cortex and Hippocampus of the Adult Mouse Brain

Several lines of investigation revealed that JNK plays important roles in neuronal inflammation under in vivo conditions. Furthermore, it has reported that increased p-JNK activation mainly disturb the antioxidant defense mechanism and subsequently initiates neuroinflammatory and neurodegenerative cascades [15, 36, 37]. To evaluate the chronic ischemic condition effects on neuroinflammation, we analyzed the activated p-NF-κB and the expression level of proinflammatory cytokines such as TNF-α, IL-1β, and NOS2 in the ipsilateral cortex and hippocampus of the ischemic brain. Our Western blot results demonstrated that chronic ischemic conditions significantly enhanced gliosis and elevates the expression level of p-NF-κB, TNF-α, IL-1β, and NOS2 in the ipsilateral cortex and hippocampus of the ischemic mouse. On the other hand, treatment with SP600125 for 7 days alleviated activated gliosis and reduced the expression level of neuroinflammatory mediators in the abovementioned regions of the ischemic mouse brain (Figures 3(a) and 3(b)). Furthermore, immunofluorescence results supported our immunoblot results in the chronic ischemic conditions markedly induced the glial, elevated the expression level of p-NF-κB, overactivation of GFAP, and increased immunoreactivity of TNF-α in the ipsilateral side of the ischemic brain compared to saline-treated control mice. However, treatment with SP600125 significantly reversed the effects of chronic ischemic conditions while reducing gliosis and inhibiting the neuroinflammatory mediators in mouse brain (Figures 3(c)–3(f)). These results suggested that when there is elevated oxidative stress in neuronal cells, it activates stress kinase JNK which subsequently triggers p-NF-κB-mediated gliosis, which further promotes the release of neuroinflammatory mediators.

Figure 3

Chronic ischemic condition increases inflammatory responses in ipsilateral cortex and hippocampus of the adult mouse. (a, b) Showing the western blot results of inflammatory mediators. (c-f) Results immunofluorescence results of inflammatory cytokines. P value = P < 0.05, #P < 0.05. Scale bar = 50 μm while magnification was 10x for confocal.

3.4. Chronic Ischemic Condition Increases Apoptotic Neurodegeneration

In agreement with previous studies, the p-JNK and inflammatory responses mediate neuronal apoptosis and degeneration. Furthermore, it has been reported that cerebral ischemia decreases mitochondrial membrane potential and increases the release of cytochrome c (Cyto. C), which in turn activates caspases cascade, which further induces apoptotic neurodegeneration [38, 39]. Herein, we determined the effects of chronic ischemia on neuronal apoptosis. Our Western blot results indicated that chronic cerebral ischemia remarkably increased the expression of apoptotic markers such as Bax, Cyto. C, caspase-3, and PARP-1 (DNA damage markers) and decreased the expression of antiapoptotic marker Bcl-2 in the ipsilateral cortex and hippocampus of ischemic mice. However, SP600125 treatment significantly decreased the expression of neuronal apoptotic markers, highlighting the significant role of JNK in apoptotic neurodegeneration (Figures 4(a) and 4(b)). Furthermore, our immunofluorescence results also revealed an increased immunoreactivity of the apoptotic markers, such as Bax and Caspase-3 in the cortex, hippocampal (CA1 regions) of the ischemic mice as compared to the control group. Interestingly, treatment with SP600125 regulated the mitochondrial system and decreased the apoptotic markers in the isch+SP600125 mice group (Figures 4(c) and 4(d)). To further evaluate the effect of SP600125 on neuronal apoptosis, we performed an in vivo FJB staining. Our results indicated an increased number of FJB+ve cells (dead cells) in the cortex and hippocampal CA1 region of the ischemic group. However, SP600125 treatment reversed these effects and reduced the level of dead neuronal cells (Figure 4(e)). Our Nissl staining also revealed that the number of viable cells in the ischemic mouse brain was significantly reduced as compared to the saline-treated mouse brain. Conversely, SP600125 treatment significantly increased the number of viable cells in the isch+SP600126 group (Figure 4(f)). Overall, these results suggest that the inhibition of JNK regulates the apoptotic neurodegeneration in chronic ischemic conditions.

Figure 4

Chronic ischemic condition upregulates neuronal apoptosis and neurodegeneration. (a, b) Western blot results of apoptosis-related markers. β-Actin was used as a loading control and ∗P < 0.05, #P < 0.05. Density values are expressed in arbitrary units (A.U). (c, d) Results of immunofluorescent analysis for apoptotic markers. (e) Showing FJB results while (f) Specify nissl staining results. The nissl staining result magnification was 20x. GraphPad Prism 6.0 soft was used to determine the significance level.

3.5. Chronic Ischemic Condition Decreases Synaptic Protein Expression in the Hippocampus of Adult Mice

JNK signaling mechanisms have been implicated in various synaptic and memory dysfunction disorders [40, 41]. Another study demonstrated that ischemic stroke could damage hippocampal neurons, which results in memory impairment [42]. To evaluate the effects of chronic cerebral ischemia on synaptic proteins, we performed Western blot and immunofluorescence assay. Our results indicated that chronic CCA ligation reduces the expression level of synaptic proteins such as PSD95 and SNAP25. On the other hand, SP600125 significantly restored the deregulated expression levels of the synaptic markers in the isch+SP600125-treated mouse brain (Figures 5(a)–5(c)). Next, we performed MWM and Y-Maze tests for learning/memory behavior. In the MWM test, our results demonstrated that the chronic ischemic mice showed memory deficits as indicated by increased latency time (time taken by the mice to reach the platform), a decrease in the number of platform crossings, and less time spent in the target quadrant during the probe test, i.e., on the 5th day without a platform. However, SP600125 treatment reversed these deficits, reducing the latency time, and increasing the number of crossings and time spent on the targeted quadrant (Figures 5(d), 5(e), and 5(h)). Next, we performed the Y-Maze test to evaluate spatial working memory. Our findings revealed that ischemic mice showed a lower percentage of spontaneous alternation compared to the saline-treated control group. However, SP600125 significantly regulated the spontaneous alternation percentage in the isch+SP600125 treated group (Figure 5(i)).

Figure 5

Chronic ischemic condition decreases synaptic proteins and induces memory dysfunction. (a) Represents the western blot results of synaptic markers hippocampus of adult mice. (b, c) Immunofluorescence analysis of synaptic proteins. (d, h, e, i) Indicating the water maze and Y-maze results while (f, g) specify the water maze and Y-maze trajectories, respectively.

4. Discussion

Many studies have shown that stress-associated kinase, JNK, is activated because of oxidative stress and increased ROS burden in a cerebral ischemic mouse model [43, 44]. However, their pattern of activation is still not well-reported. This may depend upon the severity of the stroke, vulnerability of ischemic tissue, and the time of hypoperfusion. Inhibition of active JNK in cerebral ischemia through pharmacological methods needs wide-ranging exploration. Herein, according to our knowledge, we investigated the effects of unilateral cerebral ischemia on JNK phosphorylation and oxidative stress, as there are very few reports regarding the activation and persistence of activated and JNK in the case of chronic ischemia. We also demonstrated that the inhibition of active JNK and its downstream Nrf2 and NF-κB signaling through a specific JNK inhibitor SP600125 might alleviate neuronal apoptosis, neuroinflammation, and memory disorders in a model of unilateral cerebral ischemia. These current findings emphasizing the possibility that Nrf2 induced neuroprotective effects may be achieved by the instigation of antioxidant mechanisms and downregulation of inflammatory mediators triggered by the NF-κB pathway. The findings are consistent with previous studies, suggesting that the activation of NF-κB could be reversed by the Nrf2 activators, such as Dimethylfumerate [45]. Similarly, the suppression of NF-κB has shown promising effects on the activation of Nrf2 [46]. The above findings highlighting the crosstalk between Nrf2 and NF-κB in the SP600125-induced neuroprotection in the hypoperfusion-induced neurodegenerative conditions.

Oxidative stress has long been considered for its critical role in the deregulation of multiple signaling pathways. Oxidative stress in the ischemic brain has been considered the main contributor to detrimental effects which lead to brain degeneration. Studies have shown that ischemic injuries increase oxidative stress in neuronal cells [47, 48]. From both histological and immunoblot analysis, it was revealed that oxidative stress was sustained for up to 28 days following ischemic injury in the cortex and hippocampus of the mouse brain. Several studies have suggested that nuclear translocation of Nrf2 and the expression of its target gene HO-1 provoke an antioxidant system, which protects the brain cells against oxidative stress [15, 49]. A large body of evidence has shown that accumulated ROS burden increases the level of active p-JNK and downregulates the Nrf2/HO-1 protein level [50-52]. We therefore sought to investigate the correlation between stress-associated active JNK and Nrf2/HO-1 signaling in chronic ischemic mouse brain. Interestingly, these chronic oxidative stress conditions deregulated the endogenous master antioxidant Nrf2/HO-1 signaling in the chronic ischemic mouse brain. However, treatment of SP600125 significantly regulated the Nrf2/HO-1 signaling pathway in chronic ischemic mouse brain, suggesting a possible correlation between active JNK and the Nrf2/HO-1 signaling pathway. Our results supported the hypothesis and possibilities that oxidative stress associated with activated JNK might be implicated in the downregulation of Nrf2/HO-1 pathways.

Multiple well-reported studies support a correlation between brain oxidative stress and neuroinflammation during ischemic stroke [37, 53]. Further, a huge number of studies have also demonstrated that increased oxidative stress conditions disturbed the brain homeostasis and promoted neuroinflammation in other neurodegenerative diseases, for example, AD (Alzheimer's disease) and PD (Parkinson's disease) [54]. It has been reported that p-JNK is an upstream of the inflammatory cascade and involved in the activation of various inflammatory and apoptotic signaling pathways [16]. Furthermore, we verified that an essential cascade of oxidative stress and neuroinflammation turned on during ischemic shock. Our results showed that CCA ligation provokes neuroinflammatory responses, activates microglia, and turns on the release of proinflammatory cytokines such as p-NF-κB, TNF-α, IL-1β, and NOS2 in ischemic mouse brain, which is consistent with previous studies [55, 56]. It was interesting to show that SP600125 treatment significantly abrogated these neuroinflammatory cascades. These reported studies and our finding recommended that inhibition of stress associated with JNK in chronic oxidative stress and accumulated ROS-associated neuroinflammation would be a key therapeutic target.

Recently, it has been reported that ischemic stroke could initiate a mitochondrial apoptotic pathway [57, 58]. More interestingly, other studies have reported that an increased level of ROS plays a critical role in tissue damage and neuronal apoptosis after cerebral ischemia [59, 60]. Previous literature reviews reported that ischemic damage results in an early response in gene expression of Bax and p53, which further releases molecules like cytochrome c, resulting in cell death [61, 62]. Moreover, it has been reported that unilateral ischemia (common carotid artery occlusion) for 4–8 h along with hypoxia resulted in moderate-to-higher level of ischemic pathological alterations in the ipsilateral hippocampus, cortex, and striatum in 91% of the animals and infarction in 56% of the brains, thus, causing neuronal degeneration in the brains [63]. Several reports suggest that the release of cytochrome c into the cytoplasm activates caspase cascades which leads to cleaving the poly (ADP-ribose) polymerase (PARP-1) protein [64, 65]. Cleavage of PARP-1 causes DNA damage and neuronal cell death [66, 67]. Our study revealed that long-term neuronal hypoxia induces sustained mitochondrial system deregulation and activations of apoptotic markers such as Bax, cytochrome c, PARP-1, and caspase-3 and reduces the expression of antiapoptotic Bcl-2 protein in the ipsilateral cortex and hippocampus of the ischemic mouse brain. We also showed that SP600125 significantly reduced the levels of these apoptotic markers and promoted the expression of antiapoptotic Bcl-2 level. To further support our findings, we conducted FJB and Nissl staining to evaluate the effects of chronic ischemia on neurodegeneration. Chronic ischemia markedly promoted neurodegeneration in the brains. In contrast, SP600125 significantly ameliorated the ischemic detrimental effects of neurodegeneration.

Recently, it has been well-established that synaptic proteins are involved in memory dysfunction and cognitive impairment in ischemic conditions [68-70]. Ischemic disorders and their consequences have long been identified as playing key roles in the pathology and deregulation of synaptic (SNAP25, PSD95) proteins and memory impairments [42, 71, 72]. Interestingly, our findings also supported the above literature and indicated that not only acute stroke results in a decrease in the expression of synaptic markers but chronic ischemic condition also results in the downregulation of SNAP25 and PSD95 proteins. As mentioned previously, the ischemic brain injury is responsible for the progressive loss of memory functions, we used MWM and Y-Maze tests. Our results demonstrated that chronic ischemic condition induces cognitive deficits, as observed in MWM and Y-Maze. However, ischemic mice treated with stress kinase JNK inhibitor SP600125 showed a significant improvement in memory deficit.

5. Conclusion

Our current research work provides convincing evidence that insufficient blood supply to the brain for a very long time induces chronic oxidative stress-induced JNK overactivation, which further mediates neuronal cell death and cognitive impairments via dysregulation of Nrf2/HO-1 signaling. Our study also concludes that the inhibition of stress associated with JNK through its specific inhibitor SP600125 reduces neuroinflammation, neurodegeneration, and regulates cognitive dysfunction in chronic ischemic mouse model (Figure 6).

Figure 6

Indicating the diagrammatic abstract.