Triad1 Promotes the Inflammatory Response and Neuronal Apoptosis to Aggravate Acute Spinal Cord Injury in Rats.

Objective: Spinal cord injury (SCI) is one of the most devastating central injuries, resulting in serious locomotor deficits. Triad1 is known to play an important role in SCI, but its effects on the inflammatory response and physiological behavior have not been thoroughly studied. This study is aimed at examining the effects of Triad1 on the inflammatory response and neuronal injury in acute SCI in rats.
Methods: Twenty-four male Sprague-Dawley (SD) rats were randomly divided into a control group, SCI group, sh-NC group, and Triad1 knockout group (sh-Triad1). The Basso Beattie Bresnahan locomotor rating scale was utilized for the assessment of the motor ability of rats. Hematoxylin and eosin (H&E), Luxol fast blue (LFB), and TUNEL staining were used to assess the pathological injury, demyelination, and neuronal apoptosis, respectively. ELISA was used to detect the levels of IL-1β, IL-10, and TNF-α, and qRT-PCR was used to examine the expression level of Triad1. Furthermore, the protein levels of Triad1, Bax, Bcl-2, and cleaved caspase-3 were determined using western blotting.
Results: The Triad1 expression level was upregulated in damaged spinal cord tissue. Knockdown of Triad1 improved motor function and reduced SCI as well as apoptosis of spinal cord neurons. In addition, the knockdown of Triad1 inhibited the inflammatory response caused by SCI.
Conclusion: Knockdown of Triad1 can reduce SCI in rats with acute SCI by inhibiting the inflammatory response and apoptosis.

1. Introduction

Spinal cord injury (SCI) is a fatal central nervous system disease with high morbidity and disability rates [1], which not only causes great pain to patients but also places a severe economic burden on society [2]. Approximately 2.5 million people in the United States are reported to be affected by SCI, with more than 130,000 new cases per year [3]. Meanwhile, the incidence of spinal injury in China ranges from 25 to 35 cases/100,000, and approximately 1/7-1/6 of the cases are accompanied by SCI [4]. The mechanism of SCI is divided into primary injury and secondary injury [5]. Specifically, primary injury is irreversible, while secondary injury is different. As an active regulatory process at the cellular and molecular levels, secondary injury has reversible and controllable properties [6]. Therefore, the study of secondary injury is significant. Some studies have reported that the inflammatory response is considered to be an important link triggering the secondary injury cascade [7-9]. In addition, large-scale immune responses caused by SCI may result in apoptosis of neurons and oligodendrocytes [10, 11]. Moreover, neuronal apoptosis after SCI not only is a key component of the secondary injury process but also plays a major role in neurological dysfunction development [12]. In recent years, increasing attention has been given to the potential value of inflammation and neuronal apoptosis as well as their interaction in the pathological study of SCI [13, 14].

Triad1 (ARIH2, ariadne RBR E3 ubiquitin protein ligase 2), an E3 ubiquitin ligase, is an important member of the ubiquitin proteasome system (UPS) family [15]. Studies have shown high Triad1 expression in the process of granulocyte and monocyte differentiation [16, 17]. Additionally, Triad1 is involved in bone marrow colony formation and cell cycle regulation by interacting with its binding proteins [15, 18]. Triad1 can interact with EH domain-containing protein 1 (EHD1) to further regulate the neurite outgrowth of neurons after SCI [19]. Moreover, it has also been indicated that Triad1 can promote neuronal apoptosis after SCI by regulating the p53-Caspase3 pathway [12]. However, there are few studies on the effects of Triad1 on the inflammatory response and the biological behavior of neural cells in spinal cord central nervous system injury. Therefore, in this experiment, an in vitro ALLEN SCI model was constructed to investigate the effects of Triad1 on the inflammatory response and neuronal cell biological behavior in rats with SCI. The objective of this study was to identify potential therapeutic targets for SCI and to provide a theoretical basis and new ideas for clinical medicine.

2. Materials and Methods

2.1. Experimental Animals

Twenty-four SPF grade Sprague–Dawley (SD) male rats (age: 6-8 weeks; weight: 180-220 g) were provided by Beijing Vital River Laboratory Animal Technology Co., Ltd. All rats were housed in a 12/12 h dark/light environment at a temperature of 22°C and a relative humidity of 60%. Follow-up experiments were conducted after 7 days of adaptive feeding. This study was approved by the Ethics Committee of Yangzhou Hongquan Hospital.

2.2. Establishment and Treatment of SCI Model of Rats

The model was constructed according to the previous study [20]. Specifically, the rats were anesthetized by intraperitoneal injection of pentobarbital sodium (Chinese PLA General Hospital) and shaved centered on T10. Then, routine disinfection was performed, and a surgical incision was made to expose the T9-T11 spinous process and the T10 thoracic spine. Subsequently, the T10 thoracic spinal cord was hit by a 10 g impaction rod from a 2.5 cm free fall. After hitting, the impaction rod was removed. The appearance of the rat's spasmodic swing in the tails as well as retraction fluttering in both hind limbs and trunk at the moment of injury were regarded as successful modeling.

Twenty-four SD male rats were randomly divided into 4 groups (6 rats per group): control group: only laminectomy was performed on rats; SCI model group; sh-NC group: after the establishment of the SCI model, the rats were injected intrathecally with lentiviruses (Shanghai Sangon Biotech Co., Ltd.) containing negative shRNA once a day, 50 μL/rat, for 3 consecutive days; sh-Triad1 group: SCI rats were injected intrathecally with lentivirus containing Triad1 shRNA, once a day, for 3 consecutive days; and finally, spinal cord tissue was taken from the injured position of rats in the SCI group 12 h, 1 day, 3 days, and 7 days after SCI modeling.

2.3. Assessment of Hindlimb Motor Function

The Basso Beattie Bresnahan (BBB) locomotor rating scale was utilized to assess hindlimb motor function in rats. Scale of marks was as follows: part 1 (0-7 points) represented the evaluation of the movement of each joint in the hind limbs, part 2 (8-13 points) represented the evaluation of the gait and coordination of the hind limbs, and part 3 (14-21 points) represented the assessment of fine movements of the paws during animal movement. A double-blind method was adopted, and two people were trained to independently finish each assessment. Then, the average value was taken.

2.4. H&E Staining

The spinal cord tissues in each group of rats on the third day after SCI modeling were fixed with 4% paraformaldehyde for 48 h. After routine paraffin embedding, the tissues were prepared for serial sections at approximately 4 μm thick. Subsequently, the sections were stained with hematoxylin and eosin, histopathological alterations were assessed, and SCI was scored.

2.5. Luxol Fast Blue (LFB) Staining

The spinal cord tissues of rats in each group on the third day after modeling were collected to prepare paraffin sections. Then, the sections were attached to glass slides, dried at 37°C, rinsed with 0.01 mol/L PBS, placed in LFB for staining at 37°C overnight, washed with 95% alcohol, and then transferred to distilled water for washing. Furthermore, after the sections were differentiated for 10 s by 0.05% lithium carbonate, distilled water was utilized to wash the sections and terminate the differentiation. Then, the sections were subjected to 70% fine differentiation until the gray and white matter were clearly distinguished. Afterwards, the sections were dehydrated, cleared, and sealed after adequate washing with water. The degree of injury and demyelination of rat spinal cord tissue was assessed under a light microscope. Areas covered by LFB staining were quantified using Image-Pro Plus software (Media Cybernetics, MD, USA) [21].

2.6. TUNEL Staining

Routine paraffin sections were performed on spinal cord tissues, and neuronal apoptosis and its changes in spinal cord tissues were observed through TUNEL staining [22]. Specifically, staining was performed according to the instructions of the TUNEL Apoptosis Kit (Jiancheng Bioengineering Institute, Nanjing, China). In addition, the occurrence of tan on the nucleus and nuclear membrane after staining was considered a criterion for determining positive cells. The number of positive cells in five random fields was counted under a light microscope for each section and averaged.

2.7. Enzyme-Linked Immunosorbent Assay (ELISA)

The levels of IL-1β, IL-10, and TNF-α in the spinal cord tissues of injured rats in each group on the third day after modeling were measured using an ELISA kit in strict accordance with the kit instructions.

2.8. qRT–PCR

Total RNA from brain tissue was extracted using the TRIzol kit (Thermo Fisher Scientific, USA) and then reverse transcribed into cDNA with a reverse transcription kit (Takala, Japan). QRT–PCR was subsequently performed using the SYBR ® Premix Ex TaqTMII kit (TaKaRa, Japan). β-Actin was used as an internal reference gene. The relative transcription level of the target gene was calculated by the relative quantification method (2-△△CT method). Three duplicate wells were set for each sample, and each experiment was repeated three times. The primer sequences are shown in Table 1.

Table 1

Primer sequences.

Primers	Sequences
Triad1	F: 5′-CCTGTCAGCACCAGTTTTGC-3′
	R: 5′-ACGCTCTAGGCTCCTGTACCC-3′
β-Actin	F: 5′-GGATTCCTATGTGGGCGACGA-3′
	R: 5′-CGCTCGGTGAGGATCTTCATG-3′

2.9. Western Blot

The protein concentration was determined using a BCA assay kit (Thermo Fisher Scientific, USA) after protein extraction from rat spinal cord tissue. Specifically, 5 μL of 5× SDS PAGE protein loading buffer was added and boiled for the subsequent trial. After SDS PAGE electrophoresis, the proteins were transferred to PVDF membranes. Then, the membranes were blocked with blocking solution containing 5% nonfat dry milk for 1 h, and primary antibodies were added for incubation overnight at 4°C. Subsequently, the membranes were shaken and rinsed with PBST three times. Furthermore, the diluted secondary antibody was added for a 2-h incubation at 4°C. After washing, the ECL luminescence agent was evenly dripped into the membranes, and the FliorchemHD2 imaging system was applied for scanning analysis.

2.10. Statistical Analysis

SPSS 21.0 statistical software was used to perform the statistical analysis of the data in the current study. A t-test was used for comparisons between two groups, and one-way analysis of variance was used for comparisons between multiple groups. Experimental data are expressed as the mean ± standard deviation (SD), and P < 0.05 was considered statistically significant.

3. Results

3.1. Knockdown of Triad1 Improves Motor Function and Reduces SCI in Rats

The Triad1 expression level in the damaged spinal cord was first detected. The results showed that compared with the control group, the mRNA and protein expression levels of Triad1 in the spinal cord tissue of the injured site of rats in the SCI group were significantly increased at 12 h, 1 day, 3 days, and 7 days after SCI modeling. Moreover, the mRNA and protein expression levels of Triad1 gradually increased over time, peaked on day 3, and significantly decreased on day 7 (Figures 1(a) and 1(b)). Further BBB score results showed a significant decrease in BBB score in the SCI model group compared with the control group, while knockdown of Triad1 caused a notable increase in the BBB score (Figure 1(c)). The results of H&E staining showed the normal spinal cord tissue structure in the control group: blurred boundary between white matter and gray matter, visible necrotic tissue and cell debris, significantly decreased number of neurons, and obvious inflammatory cell infiltration in the SCI group and sh-NC group. In addition, improved spinal cord structure, neurons with better histological features, and reduced inflammatory cell infiltration were observed in the sh-Triad1 group (Figure 1(d)). Furthermore, the results of LFB staining revealed that the LFB-positive area was significantly reduced, and the degree of demyelination was increased in the SCI model group compared with the control group. However, after knockdown of Triad1, the LFB-positive area was significantly increased, and the degree of demyelination was decreased (Figure 1(e)). These findings indicate that the knockdown of Triad1 effectively ameliorated motor dysfunction and reduced SCI in the rats.

Figure 1

Effect of Triad1 knockdown on spinal cord injury in rats. (a) Triad1 mRNA expression level in the spinal cord tissue of the injured site of rats in each group by qRT–PCR. (b) Triad1 protein expression level in the spinal cord tissue of the injured site of rats in each group by western blot; ∗∗P < 0.01vs. 0 d group. (c) BBB scale showing the motor ability of rats in each group before and after modeling. (d) H&E staining for the observation of the pathological changes in spinal cord tissues on the third day after modeling in each group; blue arrows indicate inflammatory cells and green arrows indicate neuronal cells. (e) LFB staining for observing the degree of demyelination of spinal cord tissues on the third day after modeling in each group. The histogram represents the average Luxol fast blue burden observed. ∗∗P < 0.01 vs. control group, ##P < 0.01 vs. sh-NC group.

3.2. Knockdown of Triad1 Reduces Apoptosis of Spinal Neurons in SCI Rats

The effect of Triad1 knockdown on spinal cord neuronal apoptosis in SCI rats was further examined. The results showed a remarkable increase in neuronal apoptosis and the number of TUNEL-positive cells in the SCI model group compared with the control group; however, both neuronal apoptosis and the number of TUNEL-positive cells were obviously decreased after knockdown of Triad1 (Figure 2(a)). Furthermore, the western blot results showed that the expression levels of the apoptosis-related proteins Bax and cleaved caspase-3 increased significantly, and that the expression level of Bcl-2 decreased obviously in the model group. After knockdown of Triad1, the expression levels of Bax and cleaved caspase-3 decreased notably and the expression level of Bcl-2 increased significantly (Figure 2(b)). The above results suggested that knockdown of Triad1 could reduce apoptosis of spinal cord neurons in SCI rats.

Figure 2

Effect of Triad1 knockdown on the apoptosis of spinal cord neurons in SCI rats. (a) TUNEL staining assessed neuronal apoptosis in the spinal cord tissues of rats in each group on the third day after modeling. (b) Western blot detected the protein expression levels of Bax, cleaved caspase-3, and Bcl-2 in the spinal cord tissues of injured sites of rats in each group, ∗∗P < 0.01 vs. control group, ##P < 0.01 vs. sh-NC group.

3.3. Knockdown of Triad1 Improves the Inflammatory Response Induced by SCI

To investigate the effect of Triad1 on the inflammatory response induced by SCI, ELISA was applied to detect the levels of the inflammatory cytokines IL-1β and TNF-α and the anti-inflammatory cytokine IL-10. The results showed that in the spinal cord tissue of the SCI model group, the levels of IL-1β and TNF-α were significantly increased, and the level of IL-10 was obviously decreased compared to the control group. After knockdown of Triad1, the levels of IL-1β and TNF-α notably declined, and the level of IL-10 was significantly upregulated (Figures 3(a)–3(c)). These results showed that knockdown of Triad1 could improve the inflammatory response induced by SCI.

Figure 3

Effect of Triad1 knockdown on the inflammatory response induced by SCI. A-C: (a) levels of IL-1β, (b) levels of TNF-α, and (c) levels of IL-10 in the spinal cord tissues of rats in each group on the third day after modeling by ELISA. ∗∗P < 0.01 vs. control group, ##P < 0.01 vs. sh-NC group.

4. Discussion

When SCI occurs, in addition to the direct damage caused by trauma, the secondary pathological changes of cascade amplification, such as edema, hemorrhage, calcium ion influx [23], peroxidation [24], and apoptosis [14], are the key to causing difficulties in the treatment of SCI. At present, there is no effective method for the treatment and prevention of SCI caused by trauma. Nevertheless, many studies have shown that effective reduction of secondary SCI can significantly reduce the disability rate of SCI [25]. Therefore, timely prevention and treatment of secondary SCI is the key to promoting neurological recovery in patients with SCI. According to some studies, Triad1 is important in the process of many cancers. Triad1 is not only associated with many cancers, such as acute myeloid leukemia and acute promyelocytic leukemia, but also involved in the differentiation of bone marrow cells. In addition, the mRNA and protein levels of Triad1 are induced and increased during the differentiation of immature blood cells into monocytes and granulocytes [26]. The results of this study showed the upregulation of the mRNA and protein levels of Triad1 in the spinal cord tissue of a rat SCI model. The BBB locomotor rating scale (a standardized test) was used to evaluate the locomotor capacity of SCI rats in our study since SCI mainly affects motor function. The BBB score of the rat model of SCI was significantly reduced, and SCI was evident which was in accordance with the previous research [27]. The above results indicated that the hindlimb motor function and spinal cord of rats were significantly affected, and the model was successfully constructed. However, after knockdown of Triad1, the BBB score of the rat model of SCI was obviously increased, and SCI was improved, indicating a significant protective effect of knockdown of Triad1 on SCI rats.

Both spinal cord tissue edema and inflammatory cell infiltration are common pathological manifestations after SCI, and neuronal apoptosis is the main cause of impaired motor function in patients after SCI [14, 28]. Different degrees of neuronal and glial apoptosis occur in both patients and animal models at the early stage of SCI. The mitochondrial apoptosis pathway plays a major role, and Bax, Bcl-2, and cleaved caspase-3 are important regulatory proteins of the mitochondrial apoptosis pathway [29, 30]. In addition to serving as downstream substrates of caspase 3, Bcl-2 and Bax are well known to be the most important regulatory genes of apoptosis. These genes not only participate in the upstream signal transduction process but also participate in the downstream signal transduction process leading to caspase 3 activity [31]. In the current study, the examination results of the spinal cord tissue of the rat model showed a significant reduction in the number of spinal cord tissue neurons, an increase in the degree of demyelination, a rise in apoptosis, a reduction in the expression of the antiapoptotic protein Bcl-2, and a notable upregulation in the expression levels of the apoptotic proteins Bax and cleaved caspase-3. However, the knockdown of Triad1 significantly reduced the apoptosis of neurons in SCI rats. The above results showed that the knockdown of Triad1 could inhibit neuronal apoptosis after SCI and reduce SCI.

It has been reported that the inflammatory response is the main cause of SCI and secondary SCI [32, 33]. On the one hand, previous studies displayed a significant increase in the expression of the inflammatory cytokines IL-1β and tumor necrosis factor TNF-α and a notable decline in the anti-inflammatory cytokine IL-10 after SCI in rats [34]. On the other hand, the results of this study also showed that the levels of IL-1β and TNF-α were significantly increased, and the level of IL-10 was significantly decreased in the spinal cord tissue of the SCI rat model, consistent with previous findings. Nevertheless, after knockdown of Triad1, the levels of IL-1β and TNF-α were significantly upregulated, and the level of IL-10 was obviously increased in the spinal cord tissue of SCI rats. The results of this study indicated that knockdown of Triad1 could inhibit the release of proinflammatory factors, enhance the expression of anti-inflammatory cytokines, and improve the inflammatory response in a rat model of SCI.

5. Conclusion

In summary, knockdown of Triad1 can improve motor ability, reduce spinal cord tissue injury, and further inhibit the release of inflammatory factors and neuronal apoptosis in a rat model of SCI. This study provides a new strategy for the treatment of SCI.