Macrophage: A key player in neuropathic pain

Research on the relationship between macrophages and neuropathic pain has flourished in the past two decades. It has long been believed that macrophages are strong immune effector cells that play well-established roles in tissue homeostasis and lesions, such as promoting the initiation and progression of tissue injury and improving wound healing and tissue remodeling in a variety of pathogenesis-related diseases. They are also heterogeneous and versatile cells that can switch phenotypically/functionally in response to the micro-environment signals. Apart from microglia (resident macrophages of both the spinal cord and brain), which are required for the neuropathic pain processing of the CNS, neuropathic pain signals in PNS are influenced by the interaction of tissue-resident macrophages and BM infiltrating macrophages with primary afferent neurons. And the current review looks at new evidence that suggests sexual dimorphism in neuropathic pain are caused by variations in the immune system, notably macrophages, rather than the neurological system.

Deletion of CD47 from Schwann cells and macrophages hastens myelin disruption/dismantling and scavenging in Schwann cells and augments myelin debris phagocytosis in macrophages

BACKGROUND

Myelin that surrounds axons breaks in trauma and disease; e.g., peripheral nerve and spinal cord injuries (PNI and SCI) and multiple sclerosis (MS). Resulting myelin debris hinders repair if not effectively scavenged by Schwann cells and macrophages in PNI and by microglia in SCI and MS. We showed previously that myelin debris evades phagocytosis as CD47 on myelin ligates SIRPα (signal regulatory protein-α) on macrophages and microglia, triggering SIRPα to inhibit phagocytosis in phagocytes. Using PNI as a model, we tested the in vivo significance of SIRPα-dependent phagocytosis inhibition in SIRPα null mice, showing that SIRPα deletion leads to accelerated myelin debris clearance, axon regeneration and recovery of function from PNI. Herein, we tested how deletion of CD47, a SIRPα ligand and a cell surface receptor on Schwann cells and phagocytes, affects recovery from PNI.

METHODS

Using CD47 null (CD47-/-) and wild type mice, we studied myelin disruption/dismantling and debris clearance, axon regeneration and recovery of function from PNI.

RESULTS

As expected from CD47 on myelin acting as a SIRPα ligand that normally triggers SIRPα-dependent phagocytosis inhibition in phagocytes, myelin debris clearance, axon regeneration and function recovery were all faster in CD47-/- mice than in wild type mice. Unexpectedly compared with wild type mice, myelin debris clearance started sooner and CD47-deleted Schwann cells displayed enhanced disruption/dismantling and scavenging of myelin in CD47-/- mice. Furthermore, CD47-deleted macrophages from CD47-/- mice phagocytosed more myelin debris than CD47-expressing phagocytes from wild type mice.

CONCLUSIONS

This study reveals two novel normally occurring CD47-dependent mechanisms that impede myelin debris clearance. First, CD47 expressed on Schwann cells inhibits myelin disruption/dismantling and debris scavenging in Schwann cells. Second, CD47 expressed on macrophages inhibits myelin debris phagocytosis in phagocytes. The two add to a third mechanism that we previously documented whereby CD47 on myelin ligates SIRPα on macrophages and microglia, triggering SIRPα-dependent phagocytosis inhibition in phagocytes. Thus, CD47 plays multiple inhibitory roles that combined impede myelin debris clearance, leading to delayed recovery from PNI. Similar inhibitory roles in microglia may hinder recovery from other pathologies in which repair depends on efficient phagocytosis (e.g., SCI and MS).

Ferulic acid alleviates sciatica by inhibiting neuroinflammation and promoting nerve repair via the TLR4/NF-κB pathway

INTRODUCTION

Sciatica causes intense pain. No satisfactory therapeutic drugs exist to treat sciatica. This study aimed to probe the potential mechanism of ferulic acid in sciatica treatment.

METHODS

Thirty-two SD rats were randomly divided into 4 groups: sham operation, chronic constriction injury (CCI), mecobalamin, and ferulic acid. We conducted RNA sequencing, behavioral tests, ELISA, PCR, western blotting, and immunofluorescence analysis. TAK-242 and JSH23 were administered to RSC96 and GMI-R1 cells to explore whether ferulic acid can inhibit apoptosis and alleviate inflammation.

RESULTS

RNA sequencing showed that TLR4/NF-κB pathway is involved in the mechanism of sciatica. CCI induced cold and mechanical hyperalgesia; destroyed the sciatic nerve structure; increased IL-1β, IL-6, TNF-α, IL-8, and TGF-β protein levels and IL-1β, IL-6, TNF-α, TGF-β, TLR4, and IBA-1 mRNA levels; and decreased IL-10 and INF-γ protein levels and IL-4 mRNA levels. Immunohistochemistry showed that IBA-1, CD32, IL-1β, iNOS, nNOS, COX2, and TLR4 expression was increased while S100β and Arg-1 decreased. CCI increased TLR4, IBA-1, IL-1β, iNOS, Myd88, p-NF-κB, and p-p38MAPK protein levels. Treatment with mecobalamin and ferulic acid reversed these trends. Lipopolysaccharide (LPS) induced RSC96 cell apoptosis by reducing Bcl-2 and Bcl-xl protein and mRNA levels and increasing Bax and Bad mRNA and IL-1β, TLR4, Myd88, p-NF-κB, and p-p38MAPK protein levels, while ferulic acid inhibited cell apoptosis by decreasing IL-1β, TLR4, Myd88, p-NF-κB, and p-p38MAPK levels and increasing Bcl-2 and Bcl-xl levels. In GMI-R1 cells, Ferulic acid attenuated LPS-induced M1 polarization by decreasing the M1 polarization markers IL-1β, IL-6, iNOS, and CD32 and increasing the M2 polarization markers CD206, IL-4, IL-10 and Arg-1. After LPS treatment, IL-1β, iNOS, TLR4, Myd88, p-p38MAPK, and p-NF-κB levels were obviously increased, and Arg-1 expression was reduced, while ferulic acid reversed these changes.

CONCLUSION

Ferulic acid can promote injured sciatic nerve repair by reducing neuronal cell apoptosis and inflammatory infiltration though the TLR4/NF-κB pathway.

Presymptomatic macrophage targeting has a long-lasting therapeutic effect on treatment termination

Macrophage-mediated inflammation is a potent driver of disease progression in mouse models of Charcot-Marie-Tooth (CMT) 1 diseases. This leads to the possibility to consider these cells as therapeutic targets to dampen disease outcome in the so far non-treatable neuropathies. As a pharmacological proof-of-principle study, long-term targeting of nerve macrophages with the orally applied CSF-1 receptor specific kinase (c-FMS) inhibitor PLX5622 showed a substantial alleviation of the neuropathy in distinct CMT1 mouse models. However, regarding translational options, clinically relevant questions emerged regarding treatment onset, duration and termination. Corroborating previous data, we here show that in a model for CMT1B, peripheral neuropathy was substantially alleviated after early continuous PLX5622 treatment in CMT1B mice, leading to preserved motor function. However, late-onset treatment failed to mitigate histopathological and clinical features, despite a similar reduction in the number of macrophages. Surprisingly, in CMT1B mice, terminating early PLX5622 treatment at six months was still sufficient to preserve motor function at 12 months of age, suggesting a long-lasting, therapeutic effect of early macrophage depletion. This novel and unexpected finding may have important translational implications, since we here show that continuous macrophage targeting appears not to be necessary for disease alleviation, provided that the treatment starts within an early, critical time window.

Creatine promotes the repair of peripheral nerve injury by affecting macrophage polarization

The present study aimed to explore whether creatine promotes the repair of peripheral nerve injury and its possible mechanism. In vitro: RAW264.7 cells were used to investigate the role of proteins related to the JAK2/STAT1 pathway in the polarization of macrophages treated with creatine. In vivo: A sciatic nerve crush model was used. After the injury, IL-4 or creatine was injected. The recovery of motor function was assessed by the rotarod test and sciatic function index at 2, 6, 10, and 16 days after injury. At 16 days after injury, the ultrastructure of the nerve tissue was observed under a transmission electron microscope. Immunostaining were performed at 4 and 16 days to investigate the expression levels of macrophage-related markers as well as the distribution of macrophages after injury. Compared with the IFN-γ group, the group pretreated with creatine showed a significant decrease in p-JAK2 and p-STAT1 in vitro. The motor function of mice in the creatine group (CR₁) and creatine 4 days group (CR₂) was significantly improved compared to the control group (CON). The improvement in the CR₂ group was more significant. Immunostaining showed that infiltrating macrophages mainly comprised M1 macrophages in the CON group and M2 macrophages in the CR group. Our study shows that creatine promotes the repair of peripheral nerve injury by affecting macrophage polarization, possibly through decreasing M1 polarization by inhibiting the JAK2/STAT1 pathway.

Early targeting of endoneurial macrophages alleviates the neuropathy and affects abnormal Schwann cell differentiation in a mouse model of Charcot-Marie-Tooth 1A

We have previously shown that targeting endoneurial macrophages with the orally applied CSF-1 receptor specific kinase (c-FMS) inhibitor PLX5622 from the age of 3 months onwards led to a substantial alleviation of the neuropathy in mouse models of Charcot-Marie-Tooth (CMT) 1X and 1B disease, which are genetically-mediated nerve disorders not treatable in humans. The same approach failed in a model of CMT1A (PMP22-overexpressing mice, line C61), representing the most frequent form of CMT. This was unexpected since previous studies identified macrophages contributing to disease severity in the same CMT1A model. Here we re-approached the possibility of alleviating the neuropathy in a model of CMT1A by targeting macrophages at earlier time points. As a proof-of-principle experiment, we genetically inactivated colony-stimulating factor-1 (CSF-1) in CMT1A mice, which resulted in lower endoneurial macrophage numbers and alleviated the neuropathy. Based on these observations, we pharmacologically ablated macrophages in newborn CMT1A mice by feeding their lactating mothers with chow containing PLX5622, followed by treatment of the respective progenies after weaning until the age of 6 months. We found that peripheral neuropathy was substantially alleviated after early postnatal treatment, leading to preserved motor function in CMT1A mice. Moreover, macrophage depletion affected the altered Schwann cell differentiation phenotype. These findings underscore the targetable role of macrophage-mediated inflammation in peripheral nerves of inherited neuropathies, but also emphasize the need for an early treatment start confined to a narrow therapeutic time window in CMT1A models and potentially in respective patients.

Neuroimmune interactions and immunoengineering strategies in peripheral nerve repair

Peripheral nerve injuries result in disrupted cellular communication between the central nervous system and somatic distal end targets. The peripheral nervous system is capable of independent and extensive regeneration; however, meaningful target muscle reinnervation and functional recovery remain limited and may result in chronic neuropathic pain and diminished quality of life. Macrophages, the primary innate immune cells of the body, are critical contributors to regeneration of the injured peripheral nervous system. However, in some clinical scenarios, macrophages may fail to provide adequate support with optimal timing, duration, and location. Here, we review the history of immunosuppressive and immunomodulatory strategies to treat nerve injuries. Thereafter, we enumerate the ways in which macrophages contribute to successful nerve regeneration. We argue that implementing macrophage-based immunomodulatory therapies is a promising treatment strategy for nerve injuries across a wide range of clinical presentations.

Single-Cell Regulatory Network Inference and Clustering Identifies Cell-Type Specific Expression Pattern of Transcription Factors in Mouse Sciatic Nerve

Advances in single-cell RNA sequencing technologies and bioinformatics methods allow for both the identification of cell types in a complex tissue and the large-scale gene expression profiling of various cell types in a mixture. In this report, we analyzed a single-cell RNA sequencing (scRNA-seq) dataset for the intact adult mouse sciatic nerve and examined cell-type specific transcription factor expression and activity during peripheral nerve homeostasis. In total, we identified 238 transcription factors expressed in nine different cell types of intact mouse sciatic nerve. Vascular smooth muscle cells have the lowest number of transcription factors expressed with 17 transcription factors identified. Myelinating Schwann cells (mSCs) have the highest number of transcription factors expressed, with 61 transcription factors identified. We created a cell-type specific expression map for the identified 238 transcription factors. Our results not only provide valuable information about the expression pattern of transcription factors in different cell types of adult peripheral nerves but also facilitate future studies to understand the function of key transcription factors in the peripheral nerve homeostasis and disease.

Macrophage-specific RhoA knockout delays Wallerian degeneration after peripheral nerve injury in mice

Background

Plenty of macrophages are recruited to the injured nerve to play key roles in the immunoreaction and engulf the debris of degenerated axons and myelin during Wallerian degeneration, thus creating a conducive microenvironment for nerve regeneration. Recently, drugs targeting the RhoA pathway have been widely used to promote peripheral axonal regeneration. However, the role of RhoA in macrophage during Wallerian degeneration and nerve regeneration after peripheral nerve injury is still unknown. Herein, we come up with the hypothesis that RhoA might influence Wallerian degeneration and nerve regeneration by affecting the migration and phagocytosis of macrophages after peripheral nerve injury.

Methods

Immunohistochemistry, Western blotting, H&E staining, and electrophysiology were performed to access the Wallerian degeneration and axonal regeneration after sciatic nerve transection and crush injury in the Lyz^Cre+/−; RhoA^flox/flox (cKO) mice or Lyz2^Cre+/− (Cre) mice, regardless of sex. Macrophages’ migration and phagocytosis were detected in the injured nerves and the cultured macrophages. Moreover, the expression and potential roles of ROCK and MLCK were also evaluated in the cultured macrophages.

Results

1. RhoA was specifically knocked out in macrophages of the cKO mice; 2. The segmentation of axons and myelin, the axonal regeneration, and nerve conduction in the injured nerve were significantly impeded while the myoatrophy was more severe in the cKO mice compared with those in Cre mice; 3. RhoA knockout attenuated the migration and phagocytosis of macrophages in vivo and in vitro; 4. ROCK and MLCK were downregulated in the cKO macrophages while inhibition of ROCK and MLCK could weaken the migration and phagocytosis of macrophages.

Conclusions

Our findings suggest that RhoA depletion in macrophages exerts a detrimental effect on Wallerian degeneration and nerve regeneration, which is most likely due to the impaired migration and phagocytosis of macrophages resulted from disrupted RhoA/ROCK/MLCK pathway. Since previous research has proved RhoA inhibition in neurons was favoring for axonal regeneration, the present study reminds us of that the cellular specificity of RhoA-targeted drugs is needed to be considered in the future application for treating peripheral nerve injury.

Dorsal Root Injury-A Model for Exploring Pathophysiology and Therapeutic Strategies in Spinal Cord Injury

Unraveling the cellular and molecular mechanisms of spinal cord injury is fundamental for our possibility to develop successful therapeutic approaches. These approaches need to address the issues of the emergence of a non-permissive environment for axonal growth in the spinal cord, in combination with a failure of injured neurons to mount an effective regeneration program. Experimental in vivo models are of critical importance for exploring the potential clinical relevance of mechanistic findings and therapeutic innovations. However, the highly complex organization of the spinal cord, comprising multiple types of neurons, which form local neural networks, as well as short and long-ranging ascending or descending pathways, complicates detailed dissection of mechanistic processes, as well as identification/verification of therapeutic targets. Inducing different types of dorsal root injury at specific proximo-distal locations provide opportunities to distinguish key components underlying spinal cord regeneration failure. Crushing or cutting the dorsal root allows detailed analysis of the regeneration program of the sensory neurons, as well as of the glial response at the dorsal root-spinal cord interface without direct trauma to the spinal cord. At the same time, a lesion at this interface creates a localized injury of the spinal cord itself, but with an initial neuronal injury affecting only the axons of dorsal root ganglion neurons, and still a glial cell response closely resembling the one seen after direct spinal cord injury. In this review, we provide examples of previous research on dorsal root injury models and how these models can help future exploration of mechanisms and potential therapies for spinal cord injury repair.

Physical exercise mitigates neuropathic changes in an animal model for Charcot-Marie-Tooth disease 1X

Inherited neuropathies of the Charcot-Marie-Tooth (CMT) type 1 are still untreatable diseases of the peripheral nervous system. We have previously shown that macrophages substantially amplify neuropathic changes in various mouse models of CMT1 subforms and that targeting innate immune cells substantially ameliorates disease outcome. However, up to date, specific approaches targeting macrophages pharmacologically might entail side effects. Here, we investigate whether physical exercise dampens peripheral nerve inflammation in a model for an X-linked dominant form of CMT1 (CMT1X) and whether this improves neuropathological and clinical outcome subsequently. We found a moderate, but significant decline in the number of macrophages and an altered macrophage activation upon voluntary wheel running. These observations were accompanied by an improved clinical outcome and axonal preservation. Most interestingly, exercise restriction by ~40% accelerated amelioration of clinical outcome and further improved nerve structure by increasing myelin thickness compared to the unrestricted running group. This myelin-preserving effect of limited exercise was accompanied by an elevated expression of brain-derived neurotrophic factor (BDNF) in peripheral nerves, while the expression of other trophic factors like neuregulin-1, glial cell line-derived neurotrophic factor (GDNF) or insulin-like growth factor 1 (IGF-1) were not influenced by any mode of exercise. We demonstrate for the first time that exercise dampens inflammation and improves nerve structure in a mouse model for CMT1, likely leading to improved clinical outcome. Reducing the amount of exercise does not automatically decrease treatment efficacy, reflecting the need of optimally designed exercise studies to achieve safe and effective treatment options for CMT1 patients.

Single Cell Transcriptome Data Analysis Defines the Heterogeneity of Peripheral Nerve Cells in Homeostasis and Regeneration

The advances in single-cell RNA sequencing technologies and the development of bioinformatics pipelines enable us to more accurately define the heterogeneity of cell types in a selected tissue. In this report, we re-analyzed recently published single-cell RNA sequencing data sets and provide a rationale to redefine the heterogeneity of cells in both intact and injured mouse peripheral nerves. Our analysis showed that, in both intact and injured peripheral nerves, cells could be functionally classified into four categories: Schwann cells, nerve fibroblasts, immune cells, and cells associated with blood vessels. Nerve fibroblasts could be sub-clustered into epineurial, perineurial, and endoneurial fibroblasts. Identified immune cell clusters include macrophages, mast cells, natural killer cells, T and B lymphocytes as well as an unreported cluster of neutrophils. Cells associated with blood vessels include endothelial cells, vascular smooth muscle cells, and pericytes. We show that endothelial cells in the intact mouse sciatic nerve have three sub-types: epineurial, endoneurial, and lymphatic endothelial cells. Analysis of cell type-specific gene changes revealed that Schwann cells and endoneurial fibroblasts are the two most important cell types promoting peripheral nerve regeneration. Analysis of communication between these cells identified potential signals for early blood vessel regeneration, neutrophil recruitment of macrophages, and macrophages activating Schwann cells. Through this analysis, we also report appropriate marker genes for future single cell transcriptome data analysis to identify cell types in intact and injured peripheral nerves. The findings from our analysis could facilitate a better understanding of cell biology of peripheral nerves in homeostasis, regeneration, and disease.