SCI is a severely debilitating condition that results in significant loss of sensory and motor function (Álvarez et al., 2021; Hellenbrand et al., 2021; Walsh, Wychowaniec, Brougham, & Dooley, 2021). Extensive research has focused on identification of medications that can promote functional and pathological recovery following spinal cord injury (Jiang et al., 2021; C. Wang et al., 2018; Zheng et al., 2020). We studied a Chinese herbal medicine called Au, which has a strong therapeutic effect on SCI, and explained the mechanism by which Au promoted recovery of motor function in rats with SCI. The average hindlimb score of the SCI group was about 6 points, while the average hindlimb score in the Au group was about 12 points (Fig. 1E). These results were consistent with the general trend of the footprint analysis results (Fig. 1D). This finding demonstrated that Au promoted rehabilitation of hindlimb motor function following SCI. Furthermore, H&E staining showed that Au reduced the area of cystic cavities compared to that in the SCI group (Fig. 1F and G), showing that Au aided functional recovery following SCI.
Secondary injury, which typically manifests as severe inflammation and neuronal apoptosis (Lin et al., 2020; J. Zhang et al., 2021), is the principal pathogenic process that aggravates tissue damage and hampers recovery of motor function following SCI (C. Wang et al., 2021; C. Wu et al., 2021). Because secondary injury is a critical therapy window, several studies have focused on reducing inflammation and neuronal death following SCI to aid functional recovery (Macks, Jeong, & Lee, 2021; Pelisch, Rosas Almanza, Stehlik, Aperi, & Kroner, 2020). Microglia become over-activated if the central nervous system is injured, resulting in the release of a large number of inflammatory mediators (Boillée, 2021; Scott, Bedi, Olson, Sears, & Cox, 2021). Generation of these mediators is uncontrolled, and is accompanied by the release of neurotoxic substances such as free radicals and acute-phase proteins, and infiltration of inflammatory cells (Chen, Zhong, & Li, 2019; C. Wang et al., 2018). Inflammatory mediators and chemokines worsen secondary damage, resulting in a negative impact on recovery following SCI (Pan, Lin, Liu, & Chen, 2022; Q. Pang et al., 2021). A large number of studies have shown that reducing central nervous system inflammation and peripheral inflammatory cell infiltration (e.g., macrophage) can significantly reduce secondary damage and promote functional recovery following SCI (Gong et al., 2020; C. Wang et al., 2018). After SCI, microglia/macrophages not only polarize into M1 cells causing inflammation, but they may also partially polarize into M2 cells, which have anti-inflammatory properties (Liu et al., 2021; Xiao et al., 2021). The polarized orientation of microglia/macrophages after SCI is strongly skewed toward the M1 phenotype (Gong et al., 2020). Studies have shown that modulating the polarization of microglia/macrophages between M1 and M2 following SCI can significantly impact the inflammatory response (Lin et al., 2020; Xiao et al., 2021). Therefore, promoting M2 polarization of microglia/macrophages in the injured spinal cord may significantly improve SCI recovery. After SCI, the number of M1 phenotype microglia/macrophages increased at the injury site and permeated the surrounding area. Surprisingly, the number of M1 cells in the Au group was significantly reduced and mostly restricted to the lesion area (Fig. 2A and C), while the protein levels of the relevant M1 cell markers (CD68, iNOS, and COX-2) were also clearly reduced (Fig. 2D-H). More notably, the Au group had much greater positive numbers and protein levels of M2 microglia/macrophage than the SCI group (Fig. 2B-E). In addition, we used LPS to activate BV2 cells in vitro to mimic neuroinflammation. The results showed that LPS treatment polarized BV2 cells to the M1 phenotype and enhanced release of pro-inflammatory mediators, with a peak at 24 hours (Fig. 3). Pretreatment with Au reduced LPS-induced M1 polarization of BV2 cells and promoted M2 polarization (Fig. 4). These results were consistent with the effects of Au treatment on microglia/macrophages in vivo. We showed that Au treatment increased the M2/M1 ratio for microglia/macrophages and reduced inflammation in vivo and in vitro.
Toll-like receptor 4, a member of the toll-like receptor family, plays an important role in cellular immunity (Li, Jiang, & Wang, 2021). Studies have shown that TLR4 is abundantly expressed in microglia and is associated with pathogenesis of neuroinflammation (Wan et al., 2017). Toll-like receptor 4 can bind to various signaling proteins, such as MyD88, to activate downstream signals, resulting in immune system activation and increased expression of inflammatory genes (Jeong et al., 2014). We showed that LPS increased TLR4 and MyD88 expression in microglia. In contrast, Au clearly reversed the LPS-induced increase in TLR4 and MyD88 levels (Fig. 5A-C). These data were consistent with results showing that Au inhibited release of pro-inflammatory mediators from microglia in vitro (Fig. 4A-C). These results suggest that Au blocked the TLR4-MyD88 signaling pathway, thereby inhibiting the neuroinflammation in LPS-activated microglia. The TLR4-mediated pro-inflammatory response is also linked to a number of signaling pathways, including the NF-κB pathway (R. Li et al., 2021), which is a key component in the inflammatory response (Szatkowski et al., 2020). Studies have shown that IκBα is degraded once the NF-κB pathway is activated, which increases the phosphorylation of p65, allowing p65 to translocate from the cytoplasm to the nucleus, resulting in inflammation (White, Lin, & Hu, 2020). We found that Au not only inhibits the expression of p-IκBα, but also inhibits p65 phosphorylation and regulates its nuclear translocation (Fig. 5D, E, F, and L), indicating that Au inhibits the NF-κB pathway. To investigate whether Au suppressed LPS-induced microglial activation via NF-κB signaling, we treated microglia with PDTC, an NF-κB pathway inhibitor. The results showed that Au or PDTC therapy lowered the protein levels of p-IκBα, p-p65, iNOS, and COX2. Furthermore, co-treatment with Au and PDTC significantly reduced the expression of p-IκBα, p-p65, and inflammatory markers (e.g., iNOS and COX2) (Fig. 5G-K). Fluorescence results showed that co-incubation with Au and PDTC resulted in significantly greater translocation of p65 to the nucleus compared with that observed in response to Au or PDTC incubation alone (Fig. 5L). Our findings showed that Au exerted anti-inflammatory properties by inactivating the NF-κB pathway, which was mediated by TLR4-MyD88.
Neuronal apoptosis is an important component of secondary injury (Mortezaee et al., 2018). Inhibition of neuronal apoptosis is thought to aid in neural rehabilitation and axon regeneration (Qi et al., 2021; Y. Wu et al., 2021). Therefore, development of strategies to reduce neuronal apoptosis in SCI has received greater interest. We used TUNEL staining to measure the level of cell apoptosis after acute SCI, and the results showed that Au therapy dramatically reduced cell apoptosis (Fig. 6A and B). Furthermore, the protein level of NeuN (a neuron marker) was higher in the Au group than that in the SCI group (Fig. 6C and D). These results suggested that administration of Au after SCI inhibited neuronal apoptosis. Additionally, we found that, compared to the SCI group, the levels of the mitochondrial apoptosis-related proteins (BAX and Cleaved Caspase 3) rose in the SCI group, while the expression levels of the mitochondrial anti-apoptotic protein Bcl-2 decreased; however, this result was reversed by Au treatment (Fig. 6E and F). Cleaved caspase 3 fluorescence staining results were consistent with those obtained using western blot (Fig. 6G and H). We then used TBHP to activate PC12 cells in vitro to mimic the influence of oxidative stress on neuronal survival after SCI. CCK8 detection and live-dead labeling can both show that Au treatment clearly reversed TBHP-induced neuronal apoptosis (Fig. 7B, G and H). Additionally, after Au therapy, the amount of mitochondrial anti-apoptotic protein (Bcl-2) was partially restored (Fig. 7C-F). These results suggested that Au protected neurons from mitochondrial dysfunction-induced apoptosis.
Previous studies have shown that Au can ameliorate secondary injury by reducing inflammation and neuronal apoptosis caused after SCI. The healing and expansion of neuronal axons is facilitated by successful control of secondary injury (Pelisch et al., 2020; C. Wang et al., 2021), and axon regeneration is thought to play a critical role in the recovery of motor function following SCI (C. Wang et al., 2019; Zhou et al., 2020). Therefore, we evaluated axonal recovery following SCI. Our findings showed that the expression of positive MAP2 axon structural proteins was higher, the region was more densely populated with axons, and axonal organization was more regular in the Au group than in the SCI group (Fig. 8B and C). We used immunofluorescence to stain GAP43 (a neurofilament marker), and showed that Au therapy increased GAP43-positive fibers in the lesion margin of SCI and extended toward the lesion center (Fig. 8D and E). When compared to the SCI group, MAP2 and GAP43 protein levels were partially restored in the Au group (Fig. 8F-H). These results indicated that treatment with Au stimulated regeneration of MAP2-positive axons and GAP43-positive neurofilaments by preventing secondary injury after SCI.
There were several limitations to our study. First, the long-term negative effects of Au injections are unknown. Second, Au is believed to exert a strong anti-inflammatory effect and to promote decreased expression of inflammatory mediators released by inflammatory cells via a variety of mechanisms, but additional mechanisms were not evaluated in this study. Furthermore, because several Au receptors have yet to be identified, it is uncertain whether the anti-inflammatory effects on different phenotypes of inflammatory cells and neuroprotective effects on neurons are mediated by different receptors.