Toll-Like Receptor 4 (TLR4) Expression Affects Schwann Cell Behavior in vitro

Toll-Like Receptor 4 (TLR4) Promotes DRG Regeneration and Repair after Sciatic Nerve Injury via the ERK-NF-kB Pathway

Previously, we found that the expression of Toll-like receptor 4 (TLR4) is altered after sciatic nerve injury, and its differential expression plays a key role in recovery. However, the mechanisms by which TLR4 affects neuronal function in the dorsal root ganglion (DRG) have not been completely evaluated. The objective is to determine TLR4 expression in DRG tissues after sciatic neural injury and exploring the effects of TLR4 knockdown and overexpression in the DRG on neuronal function and nerve regeneration in rats in vivo and in vitro. We established a model of nerve injury and utilized molecular biology and cell biology experiments to explore the molecular mechanisms by which TLR4 in the DRG affects sciatic nerve restoration and regeneration after injury. Verified the localization of TLR4 in DRG neurons. Investigated pathways that related to apoptosis or nerve regeneration by which TLR4 regulates the function of DRG neurons. TLR4 expression was upregulated in the DRG tissues of rats after sciatic nerve injury. TLR4 overexpression promoted axon regeneration and inhibited apoptosis in DRG neurons. TLR4 promoted the regeneration of axons and the recovery of motor and sensory functions in the sciatic nerve after injury in vivo, and the data showed that TLR4 may regulate the function of DRG neurons and promote nerve repair and regeneration through the ERK and NF-κB signaling pathways in vivo and ex vivo. The study suggests that TLR4 may regulate the function of DRG neurons and promote nerve regeneration by affecting the ERK and NF-κB signaling pathways.

Hyperglycaemia Aggravates Oxidised Low-Density Lipoprotein-Induced Schwann Cell Death via Hyperactivation of Toll-like Receptor 4

Increased low-density lipoprotein levels are risk factors for diabetic neuropathy. Diabetes mellitus is associated with elevated metabolic stress, leading to oxidised low-density lipoprotein formation. Therefore, it is important to investigate the mechanisms underlying the pathogenesis of diabetic neuropathy in diabetes complicated by dyslipidaemia with increased levels of oxidised low-density lipoprotein. Here, we examined the effects of hyperglycaemia and oxidised low-density lipoprotein treatment on Schwann cell death and its underlying mechanisms. Immortalised mouse Schwann cells were treated with oxidised low-density lipoprotein under normo- or hyperglycaemic conditions. We observed that oxidised low-density lipoprotein-induced cell death increased under hyperglycaemic conditions compared with normoglycaemic conditions. Moreover, hyperglycaemia and oxidised low-density lipoprotein treatment synergistically upregulated the gene and protein expression of toll-like receptor 4. Pre-treatment with TAK-242, a selective toll-like receptor 4 signalling inhibitor, attenuated hyperglycaemia- and oxidised low-density lipoprotein-induced cell death and apoptotic caspase-3 pathway. Our findings suggest that the hyperactivation of toll-like receptor 4 signalling by hyperglycaemia and elevated oxidised low-density lipoprotein levels synergistically exacerbated diabetic neuropathy; thus, it can be a potential therapeutic target for diabetic neuropathy.

Cell-free therapy based on extracellular vesicles: a promising therapeutic strategy for peripheral nerve injury

Peripheral nerve injury (PNI) is one of the public health concerns that can result in a loss of sensory or motor function in the areas in which injured and non-injured nerves come together. Up until now, there has been no optimized therapy for complete nerve regeneration after PNI. Exosome-based therapies are an emerging and effective therapeutic strategy for promoting nerve regeneration and functional recovery. Exosomes, as natural extracellular vesicles, contain bioactive molecules for intracellular communications and nervous tissue function, which could overcome the challenges of cell-based therapies. Furthermore, the bioactivity and ability of exosomes to deliver various types of agents, such as proteins and microRNA, have made exosomes a potential approach for neurotherapeutics. However, the type of cell origin, dosage, and targeted delivery of exosomes still pose challenges for the clinical translation of exosome therapeutics. In this review, we have focused on Schwann cell and mesenchymal stem cell (MSC)-derived exosomes in nerve tissue regeneration. Also, we expressed the current understanding of MSC-derived exosomes related to nerve regeneration and provided insights for developing a cell-free MSC therapeutic strategy for nerve injury.

Identification of adhesion-associated DNA methylation patterns in the peripheral nervous system

Schwann cells are unique glial cells in the peripheral nervous system. These cells provide a range of cytokines and nutritional factors to maintain axons and support axonal regeneration. However, little is known concerning adhesion-associated epigenetic changes that occur in Schwann cells after peripheral nerve injury (PNI). In the present study, adhesion-associated DNA methylation biomarkers were assessed between normal and injury peripheral nerve. Specifically, normal Schwann cells (NSCs) and activated Schwann cells (ASCs) were obtained from adult Wistar rats. After the Schwann cells were identified, proliferation and adhesion assays were used to assess differences between NSCs and ASCs. Methylated DNA immunoprecipitation-sequencing and bioinformatics analysis were used to identify and analyze the differentially methylated genes. Reverse transcription-quantitative PCR was performed to assess the expression levels of adhesion-associated genes. In the present study, the proliferation and adhesion assays demonstrated that ASCs had a more robust proliferative activity and adhesion compared with NSCs. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses were performed to identify methylation-associated biological processes and signaling pathways. Protein-protein interaction network analysis revealed that Fyn, Efna1, Jak2, Vav3, Flt4, Epha7, Crk, Kitlg, Ctnnb1 and Ptpn11 were potential markers for Schwann cell adhesion. The expression levels of several adhesion-associated genes, such as vinculin, BCAR1 scaffold protein, collagen type XVIII α1 chain and integrin subunit β6, in ASCs were altered compared with those in NSCs. The current study analyzed adhesion-associated DNA methylation patterns of Schwann cells and identified candidate genes that may potentially regulate Schwann cell adhesion in Wistar rats before and after PNI.

Sepsis Activates the TLR4/MyD88 Pathway in Schwann Cells to Promote Infiltration of Macrophages, Thereby Impeding Neuromuscular Function

INTRODUCTION

Sepsis is a kind of maladjustment response to bacterial infection and activation of coagulation, which can induce neuromuscular dysfunction. However, there is scarce of experimental evidence about the relationship between Schwann cells (SCs) and sepsis in neuromuscular dysfunction. We therefore set out to identify the potential role of SCs in sepsis-induced neuromuscular dysfunction and to explore the underlying molecular mechanism.

METHODS

Primary SCs were isolated from the left hind limb sciatic nerve of sepsis mice, which was constructed by cecal ligation and puncture. Then, the SCs were infected with adenovirus encoding toll-like receptor 4 (TLR4), MyD88, or IL-1R (with lipopolysaccharide stimulation), and the Raw 264.7 macrophages were injected with adenovirus with CCR2 silencing (with mMCP-1 stimulation). Further investigation of the interleukin 1 beta (IL-1β) and macrophage cationic peptide 1 (MCP-1) expressions, we followed reverse transcription-quantitative polymerase chain reaction and enzyme-linked immunosorbent assay techniques, the F4/80 and Ki67 expressions was observed by immunofluorescence staining, while the expressions of CCR2, FAK/p-FAK, nuclear factor-κB (NFκB)/p-NFκB, and ERK1/2/p-ERK1/2 were determined by Western blot analysis. Last, but not the least, the cell migration ability and cell proliferation ability were detected by Transwell assay and Flow cytometry respectively.

RESULTS

Our results showed that in sepsis mice, the TLR4/MyD88/ERK pathway was activated in SCs, which triggered the cells to secrete IL-1β and MCP-1. The secreted IL-1β bound with IL-1β receptor on the surface of SCs, thereby activating the IL-1β/IL-1R/MyD88/ERK pathway and further promoting the secretion of MCP-1 by SCs. MCP-1 was found to bind to CCR2 on the surface of Raw264.7 macrophages to activate the TLR4/MyD88/ERK pathway which caused the inhibition of neuromuscular function.

CONCLUSION

Sepsis significantly promotes the infiltration of macrophages by activating the TLR4/MyD88 pathway in SCs, thereby impeding neuromuscular function. Consistently, our study provides a novel concept in the area of neuromuscular dysfunction therapeutics following sepsis.

Protein Kinase Cα Promotes Proliferation and Migration of Schwann Cells by Activating ERK Signaling Pathway

Wallerian degeneration (WD) and axon regeneration generally take place following peripheral nerve injury (PNI). Schwann cells (SCs) and macrophages play major role in WD. SCs, acting as repair cells and primary signal mediators, dedifferentiate and proliferate to remove the debris, form Büngner's bands and secrete trophic factors during these processes. However, the underlying mechanisms remain poorly understood. Here, we found that protein kinase Cα (PKCα), a serine/threonine kinase, expressed in SCs was significantly up-regulated after PNI. Activating PKCα with phorbol 12-myristate 13-acetate (PMA), a phorbol ester binds and activates PKCα) promoted SCs proliferation and migration. While, silence of PKCα by siRNAs inhibited these processes. PD184352, an inhibitor of MEK1, reversed the effect induced by PMA on SCs. Mechanism studies revealed that PKCα functioned through activating the ERK signaling pathway. Furthermore, PKCα also exhibited a neuroprotective role by upregulating the expression of neurotrophic factors in SCs. To sum up, this study offers novel insights for clarifying our understanding of the involvement of PKCα in the mechanism of peripheral nerve degeneration as well as regeneration.

Claudin-15 overexpression inhibits proliferation and promotes apoptosis of Schwann cells in vitro

Our previous experiments have discovered that Claudin-15 was up-regulated in Schwann cells of the distal nerve stumps of rat models of sciatic nerve injury. However, how Claudin-15 affects Schwann cell function is still unknown. This study aimed to identify the effects of Claudin-15 on proliferation and apoptosis of Schwann cells cultured in vitro and explore the underlying mechanisms. Primary Schwann cells were obtained from rats. Claudin-15 in Schwann cells was knocked down using siRNA (siRNA-1 group) compared with the negative control siRNA transfection group (negative control group). Claudin-15 in Schwann cells was overexpressed using pGV230-Claudin-15 plasmid (pGV230-Claudin-15 group). The pGV230 transfection group (pGV230 group) acted as the control of the pGV230-Claudin-15 group. Cell proliferation was analyzed with EdU assay. Cell apoptosis was analyzed with flow cytometric analysis. Cell migration was analyzed with Transwell inserts. The mRNA and protein expressions were analyzed with quantitative polymerase chain reaction assay and western blot assay. The results showed that compared with the negative control group, cell proliferation rate was up-regulated; p-AKT/AKT ratio, apoptotic rate, p-c-Jun/c-Jun ratio, mRNA expression of protein kinase C alpha, Bcl-2 and Bax were down-regulated; and mRNA expression of neurotrophins basic fibroblast growth factor and neurotrophin-3 were increased in the siRNA-1 group. No significant difference was found in cell migration between the negative control and siRNA-1 groups. Compared with the pGV230 group, the cell proliferation rate was down-regulated; apoptotic rate, p-c-Jun/c-Jun ratio and c-Fos protein expression increased; mRNA expression of protein kinase C alpha and Bax decreased; and mRNA expressions of neurotrophins basic fibroblast growth factor and neurotrophin-3 were up-regulated in the pGV230-Claudin-15 group. The above results demonstrated that overexpression of Claudin-15 inhibited Schwann cell proliferation and promoted Schwann cell apoptosis in vitro. Silencing of Claudin-15 had the reverse effect and provided neuroprotective effect. This study was approved by the Experimental Animal Ethics Committee of Jilin University of China (approval No. 2016-nsfc001) on March 5, 2016.

MSC-Derived Exosomes-Based Therapy for Peripheral Nerve Injury: A Novel Therapeutic Strategy

Although significant advances have been made in synthetic nerve conduits and surgical techniques, complete regeneration following peripheral nerve injury (PNI) remains far from optimized. The repair of PNI is a highly heterogeneous process involving changes in Schwann cell phenotypes, the activation of macrophages, and the reconstruction of the vascular network. At present, the efficacy of MSC-based therapeutic strategies for PNI can be attributed to paracrine secretion. Exosomes, as a product of paracrine secretion, are considered to be an important regulatory mediator. Furthermore, accumulating evidence has demonstrated that exosomes from mesenchymal stem cells (MSCs) can shuttle bioactive components (proteins, lipids, mRNA, miRNA, lncRNA, circRNA, and DNA) that participate in almost all of the abovementioned processes. Thus, MSC exosomes may represent a novel therapeutic tool for PNI. In this review, we discuss the current understanding of MSC exosomes related to peripheral nerve repair and provide insights for developing a cell-free MSC therapeutic strategy for PNI.

MIF/CD74 axis participates in inflammatory activation of Schwann cells following sciatic nerve injury

Based on deep RNA sequencing of distal segments of lesioned sciatic nerves, a huge number of differentially expression genes (DEGs) were thus obtained and functionally analyzed. The inflammatory response was denoted as one of most significant biological processes following sciatic nerve injury. In the present study, ingenuity pathway analysis (IPA) demonstrated that macrophage migration inhibitory factor (MIF) was identified as a core regulator of inflammatory response through interaction with CD74 membrane receptor. By establishment of rat sciatic nerve transection model, we displayed that MIF was upregulated following sciatic nerve axotomy, in colocalization with Schwann cells (SCs). MIF promoted migration, proliferation, together with inflammatory responses of SCs in vitro. Immunoprecipitation showed that MIF interacted with CD74 receptor, through which to activate intracellular ERK and JNK signaling pathways. Interference of CD74 receptor using specific siRNA showed that the transcription of proinflammatory cytokines including TNF-α, IL-1β, as well as cytokine receptor TLR4 in SCs was significantly attenuated, supporting an participation of MIF/CD74 signal axis in SCs inflammatory response. The data provide a novel role of MIF in eliciting inflammatory response of peripheral nerve injury, which might be beneficial for precise therapy of peripheral nerve inflammation.

Intrathecal triamcinolone acetonide exerts anti-inflammatory effects on Lewis rat experimental autoimmune neuritis and direct anti-oxidative effects on Schwann cells

Background

Corticosteroids dominate in the treatment of chronic autoimmune neuropathies although long-term use is characterized by devastating side effects.

Methods

We introduce the intrathecal application of the synthetic steroid triamcinolone (TRIAM) as a novel therapeutic option in experimental autoimmune neuritis in Lewis rats

Results

After immunization with neuritogenic P2 peptide, we show a dose-dependent therapeutic effect of one intrathecal injection of 0.3 or 0.6 mg/kg TRIAM on clinical and electrophysiological parameters of neuritis with a lower degree of inflammatory infiltrates (T cells and macrophages) and demyelination in the sciatic nerve. In vitro studies in Schwann cell cultures showed an increased expression of IL-1 receptor antagonist and reduced expression of Toll-like receptor 4 after incubation with TRIAM as well as a protective effect of TRIAM against oxidative stress after H₂O₂ exposure.

Conclusion

Intrathecal TRIAM application could be a novel immunomodulatory and potentially neuroprotective option for autoimmune neuropathies with a direct effect on Schwann cells.