Involvement of mTOR Pathways in Recovery from Spinal Cord Injury by Modulation of Autophagy and Immune Response

Screening and evaluation of metabolites binding PRAS40 from Erxian decoction used to treat spinal cord injury

Objective: The PRAS40 is an essential inhibitory subunit of the mTORC1 complex, which regulates autophagy. It has been suggested that Erxian Decoction (EXD) could treat spinal cord injury (SCI) via the autophagy pathway. However, the mechanism of whether EXD acts through PRAS40 remains unclear. Methods: With the help of immobilized PRAS40, isothermal titration calorimetry (ITC) and molecular docking, the bioactive metabolites in the EXD were screened. To establish in vitro SCI models, PC12 cells were exposed to hydrogen peroxide (H2O2) and then treated with the identified EXD substances. Furthermore, Western blot assay was carried out to identify potential molecular mechanisms involved. For assessing the effect of metabolites in vivo, the SCI model rats were first pretreated with or without the metabolite and then subjected to the immunohistochemistry (IHC) staining, Basso, Beattie & Bresnahan (BBB) locomotor rating scale, and H&E staining. Results: The immobilized PRAS40 isolated indole, 4-nitrophenol, terephthalic acid, palmatine, sinapinaldehyde, and 3-chloroaniline as the potential ligands binding to PRAS40. Furthermore, the association constants of palmatine and indole as 2.84 × 106 M-1 and 3.82 × 105 M-1 were elucidated via ITC due to the drug-like properties of these two metabolites. Molecular docking results also further demonstrated the mechanism of palmatine binding to PRAS40. Western blot analysis of PC12 cells demonstrated that palmatine inhibited the expression of p-mTOR by binding to PRAS40, activating the autophagic flux by markedly increasing LC3. The injection of palmatine (10μM and 20 μM) indicated notably increased BBB scores in the SCI rat model. Additionally, a dose-dependent increase in LC3 was observed by IHC staining. Conclusion: This research proved that EXD comprises PRAS40 antagonists, and the identified metabolite, palmatine, could potentially treat SCI by activating the autophagic flux.

A Single Injection of rAAV-shmTOR in Peripheral Nerve Persistently Attenuates Nerve Injury-Induced Mechanical Allodynia

Activation of mammalian target of rapamycin (mTOR) has been known as one of the contributing factors in nociceptive sensitization after peripheral injury. Its activation followed by the phosphorylation of downstream effectors causes hyperexcitability of primary sensory neurons in the dorsal root ganglion. We investigated whether a single injection of rAAV-shmTOR would effectively downregulate both complexes of mTOR in the long-term and glial activation as well. Male SD rats were categorized into shmTOR (n = 29), shCON (n = 23), SNI (n = 13), and Normal (n = 8) groups. Treatment groups were injected with rAAV-shmTOR or rAAV-shCON, respectively. DRG tissues and sciatic nerve were harvested for Western blot and immunohistochemical analyses. Peripheral sensitization was gradually attenuated in the shmTOR group, and it reached a peak on PID 21. Western blot analysis showed that both p-mTORC1 and p-mTORC2 were downregulated in the DRG compared to shCON and SNI groups. We also found decreased expression of phosphorylated p38 and microglial activation in the DRG. We first attempted a therapeutic strategy for neuropathic pain with a low dose of AAV injection by interfering with the mTOR signaling pathway, suggesting its potential application in pain treatment.

Pre-treatment of rapamycin transformed M2 microglia alleviates traumatic cervical spinal cord injury via AIM2 signaling pathway in vitro and in vivo

BACKGROUND

Traumatic spinal cord injury (SCI) is still devastating. It was suggested that the inhibition of mTOR may alleviate neuronal inflammatory injury but its underlying mechanism remained to be elucidated. AIM2 (absent in melanoma 2) recruits ASC (apoptosis-associated speck-like protein containing a CARD) and caspase-1 to form the AIM2 inflammasome, activate caspase-1, and elicit inflammatory responses. We designed this study to elucidate whether pre-treatments of rapamycin could suppress SCI induced neuronal inflammatory injury via AIM2 signaling pathway in vitro and in vivo.

METHODS

We performed oxygen and glucose deprivation / re-oxygenation (OGD) treatment and rats clipping model to mimic neuronal injury after SCI in vitro and in vivo. Morphologic changes of injured spinal cord were detected by hematoxylin and eosin staining. The expression of mTOR, p-mTOR, AIM2, ASC, Caspase-1 and et al were analyzed by fluorescent staining, western blotting or qPCR. The polarization phenotype of microglia was identified by flow cytometry or fluorescent staining.

RESULTS

We found BV-2 microglia without any pre-treatment cannot alleviate primary cultured neuronal OGD injury. However, pre-treated rapamycin in BV-2 cells could transform microglia to M2 phenotype and protects against neuronal OGD injury via AIM2 signaling pathway. Similarly, pre-treatment of rapamycin could improve the outcome of cervical SCI rats through AIM2 signaling pathway.

CONCLUSIONS

It was suggested that resting state microglia pre-treated by rapamycin could protect against neuronal injury via AIM2 signaling pathway in vitro and in vivo. Pre-inhibition of mTOR pathway may improve neuronal protection after SCI.

Therapeutic potential of natural compounds from herbs and nutraceuticals in spinal cord injury: Regulation of the mTOR signaling pathway

Spinal cord injury (SCI) is a disease in which the spinal cord is subjected to various external forces that cause it to burst, shift, or, in severe cases, injure the spinal tissue, resulting in nerve injury. SCI includes not only acute primary injury but also delayed and persistent spinal tissue injury (i.e., secondary injury). The pathological changes post-SCI are complex, and effective clinical treatment strategies are lacking. The mammalian target of rapamycin (mTOR) coordinates the growth and metabolism of eukaryotic cells in response to various nutrients and growth factors. The mTOR signaling pathway has multiple roles in the pathogenesis of SCI. There is evidence for the beneficial effects of natural compounds and nutraceuticals that regulate the mTOR signaling pathways in a variety of diseases. Therefore, the effects of natural compounds on the pathogenesis of SCI were evaluated by a comprehensive review using electronic databases, such as PubMed, Web of Science, Scopus, and Medline, combined with our expertise in neuropathology. In particular, we reviewed the pathogenesis of SCI, including the importance of secondary nerve injury after the primary mechanical injury, the roles of the mTOR signaling pathways, and the beneficial effects and mechanisms of natural compounds that regulate the mTOR signaling pathway on pathological changes post-SCI, including effects on inflammation, neuronal apoptosis, autophagy, nerve regeneration, and other pathways. This recent research highlights the value of natural compounds in regulating the mTOR pathway, providing a basis for developing novel therapeutic strategies for SCI.

The Role of Green Tea Catechin Epigallocatechin Gallate (EGCG) and Mammalian Target of Rapamycin (mTOR) Inhibitor PP242 (Torkinib) in the Treatment of Spinal Cord Injury

Spinal cord injury (SCI) is a devastating condition that has physical and psychological consequences for patients. SCI is accompanied by scar formation and systemic inflammatory response leading to an intense degree of functional loss. The catechin, epigallocatechin gallate (EGCG), an active compound found in green tea, holds neuroprotective features and is known for its anti-inflammatory potential. The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that exists in two functionally distinct complexes termed mTOR complex 1 and 2 (mTORC1; mTORC2). Inhibition of mTORC1 by rapamycin causes neuroprotection, leading to partial recovery from SCI. In this study the effects of EGCG, PP242 (an inhibitor of both complexes of mTOR), and a combination of EGCG and PP242 in SCI have been examined. It has been found that both EGCG and PP242 significantly improved sensory/motor functions following SCI. However, EGCG appeared to be more effective (BBB motor test, from 2 to 8 weeks after SCI, p = 0.019, p = 0.007, p = 0.006, p = 0.006, p = 0.05, p = 0.006, and p = 0.003, respectively). The only exception was the Von Frey test, where EGCG was ineffective, while mTOR inhibition by PP242, as well as PP242 in combination with EGCG, significantly reduced withdrawal latency starting from week three (combinatorial therapy (EGCG + PP242) vs. control at 3, 5, and 7 weeks, p = 0.011, p = 0.007, and p = 0.05, respectively). It has been found that EGCG was as effective as PP242 in suppressing mTOR signaling pathways, as evidenced by a reduction in phosphorylated S6 expression (PP242 (t-test, p < 0.0001) or EGCG (t-test, p = 0.0002)). These results demonstrate that EGCG and PP242 effectively suppress mTOR pathways, resulting in recovery from SCI in rats, and that EGCG acts via suppressing mTOR pathways.

The contribution of altered neuronal autophagy to neurodegeneration

Defects in cellular functions related to altered protein homeostasis and associated progressive accumulation of pathological intracellular material is a critical process involved in the pathogenesis of many neurodegenerative disorders, including Alzheimer's disease and Parkinson's disease. Autophagy is an essential mechanism that ensures neuronal health by removing long-lived proteins or defective organelles and by doing so prevents cell toxicity and death within the central nervous system. Abundant evidence has shown that neuronal autophagy pathways are altered in Alzheimer's disease, Parkinson's disease and traumas of the central nervous system including Spinal Cord Injury and Traumatic Brain Injury. In this review, we aimed to summarize the latest studies on the role that altered neuronal autophagy plays in brain health and these pathological conditions, and how this knowledge can be leveraged for the development of novel therapeutics against them.

Impairment of autophagy after spinal cord injury potentiates neuroinflammation and motor function deficit in mice

Autophagy is a catabolic process that degrades cytoplasmic constituents and organelles in the lysosome, thus serving an important role in cellular homeostasis and protection against insults. We previously reported that defects in autophagy contribute to neuronal cell damage in traumatic spinal cord injury (SCI). Recent data from other inflammatory models implicate autophagy in regulation of immune and inflammatory responses, with low levels of autophagic flux associated with pro-inflammatory phenotypes. In the present study, we examined the effects of genetically or pharmacologically manipulating autophagy on posttraumatic neuroinflammation and motor function after SCI in mice. Methods: Young adult male C57BL/6, CX3CR1-GFP, autophagy hypomorph Becn1+/- mice, and their wildtype (WT) littermates were subjected to moderate thoracic spinal cord contusion. Neuroinflammation and autophagic flux in the injured spinal cord were assessed using flow cytometry, immunohistochemistry, and NanoString gene expression analysis. Motor function was evaluated with the Basso Mouse Scale and horizontal ladder test. Lesion volume and spared white matter were evaluated by unbiased stereology. To stimulate autophagy, disaccharide trehalose, or sucrose control, was administered in the drinking water immediately after injury and for up to 6 weeks after SCI. Results: Flow cytometry demonstrated dysregulation of autophagic function in both microglia and infiltrating myeloid cells from the injured spinal cord at 3 days post-injury. Transgenic CX3CR1-GFP mice revealed increased autophagosome formation and inhibition of autophagic flux specifically in activated microglia/macrophages. NanoString analysis using the neuroinflammation panel demonstrated increased expression of proinflammatory genes and decreased expression of genes related to neuroprotection in Becn1+/- mice as compared to WT controls at 3 days post-SCI. These findings were further validated by qPCR, wherein we observed significantly higher expression of proinflammatory cytokines. Western blot analysis confirmed higher protein expression of the microglia/macrophage marker IBA-1, inflammasome marker, NLRP3, and innate immune response markers cGAS and STING in Becn1+/- mice at 3 day after SCI. Flow cytometry demonstrated that autophagy deficit did not affect either microglial or myeloid counts at 3 days post-injury, instead resulting in increased microglial production of proinflammatory cytokines. Finally, locomotor function showed significantly worse impairments in Becn1+/- mice up to 6 weeks after SCI, which was accompanied by worsening tissue damage. Conversely, treatment with a naturally occurring autophagy inducer trehalose, reduced protein levels of p62, an adaptor protein targeting cargo to autophagosomes as well as the NLRP3, STING, and IBA-1 at 3 days post-injury. Six weeks of trehalose treatment after SCI led to improved motor function recovery as compared to control group, which was accompanied by reduced tissue damage. Conclusions: Our data indicate that inhibition of autophagy after SCI potentiates pro-inflammatory activation in microglia and is associated with worse functional outcomes. Conversely, increasing autophagy with trehalose, decreased inflammation and improved outcomes. These findings highlight the importance of autophagy in spinal cord microglia and its role in secondary injury after SCI.

Protective effects of muscone on traumatic spinal cord injury in rats

Background

Traumatic spinal cord injury (SCI) is a major clinical concern, and it is a life-changing neurological condition with substantial socioeconomic implications. Muscone has been widely used in traditional Chinese medicinal formulations for its anti-inflammatory activity. However, its protective effects on traumatic SCI have not been explored. This study investigated whether muscone plays a protective role in SCI and compared its effects with those of methylprednisolone sodium succinate (MPSS).

Methods

Rats were divided into five groups: normal saline (NS; n=24), methylprednisolone (MP; w=24), and muscone 1 (MO1), muscone 2 (MO2), and muscone 3 (MO3) (n=24 in each group, collectively called the MOx groups). The SCI rat model was established by the modified Allen's method. The rats were administered muscone (MO1: 2.5 mg/kg, MO2: 5 mg/kg, and MO3: 10 mg/kg) or MP (30 mg/kg), or an equivalent volume of saline. The rats were kept under observation for 4 weeks. Malondialdehyde (MDA), superoxide dismutase (SOD), interleukin-6 (IL-6), interleukin-1β (IL-1β), and tumor necrosis factor-alpha (TNF-α) levels were detected using enzyme-linked immunosorbent assay (ELISA). The expression of glial fibrillary acidic protein (GFAP), B-cell lymphoma-2 (BCL-2), and caspase3 was detected by western blot analysis. Hematoxylin-eosin (HE), Nissl, and immunocytochemistry (ICC) staining was performed for pathological observation. Basso-Beattie-Bresnahan motor function scores were evaluated for assessment of neural functions after acute SCI.

Results

Muscone inhibited immune-inflammatory reactions, neuronal necrosis, and apoptosis. The lower limb function recovery was better in the MOx groups compared with NS and MP groups according to Basso-Beattie-Bresnahan scores. The changes were remarkable in the MO2 group compared with the other groups.

Conclusions

Muscone alleviates secondary injury after SCI by reducing immune-inflammatory reactions, neuronal necrosis, and apoptosis.

Quercetin Can Improve Spinal Cord Injury by Regulating the mTOR Signaling Pathway

The pathogenesis of spinal cord injury (SCI) is complex. At present, there is no effective treatment for SCI, with most current interventions focused on improving the symptoms. Inflammation, apoptosis, autophagy, and oxidative stress caused by secondary SCI may instigate serious consequences in the event of SCI. The mammalian target of rapamycin (mTOR), as a key signaling molecule, participates in the regulation of inflammation, apoptosis, and autophagy in several processes associated with SCI. Quercetin can reduce the loss of myelin sheath, enhance the ability of antioxidant stress, and promote axonal regeneration. Moreover, quercetin is also a significant player in regulating the mTOR signaling pathway that improves pathological alterations following neuronal injury. Herein, we review the therapeutic effects of quercetin in SCI through its modulation of the mTOR signaling pathway and elaborate on how it can be a potential interventional agent for SCI.