Macrophages in spinal cord injury: phenotypic and functional change from exposure to myelin debris

Myeloid masquerade: Microglial transcriptional signatures in retinal development and disease

Microglia are dynamic guardians of neural tissue and the resident immune cells of the central nervous system (CNS). The disease-associated microglial signature (DAM), also known as the microglial neurodegenerative phenotype (MGnD), has gained significant attention in recent years as a fundamental microglial response common to various neurodegenerative disease pathologies. Interestingly, this signature shares many features in common with developmental microglia, suggesting the existence of recycled gene programs which play a role both in early neural circuit formation as well as in response to aging and disease. In addition, recent advances in single cell RNA sequencing have revealed significant heterogeneity within the original DAM signature, with contributions from both yolk sac-derived microglia as well as bone marrow-derived macrophages. In this review, we examine the role of the DAM signature in retinal development and disease, highlighting crosstalk between resident microglia and infiltrating monocytes which may critically contribute to the underlying mechanisms of age-related neurodegeneration.

Role of inflammation in neurological damage and regeneration following spinal cord injury and its therapeutic implications

Spinal cord injury (SCI) is an incurable trauma that frequently results in partial or complete loss of motor and sensory function. Massive neurons are damaged after the initial mechanical insult. Secondary injuries, which are triggered by immunological and inflammatory responses, also result in neuronal loss and axon retraction. This results in defects in the neural circuit and a deficiency in the processing of information. Although inflammatory responses are necessary for spinal cord recovery, conflicting evidence of their contributions to specific biological processes have made it difficult to define the specific role of inflammation in SCI. This review summarizes our understanding of the complex role of inflammation in neural circuit events following SCI, such as cell death, axon regeneration and neural remodeling. We also review the drugs that regulate immune responses and inflammation in the treatment of SCI and discuss the roles of these drugs in the modulation of neural circuits. Finally, we provide evidence about the critical role of inflammation in facilitating spinal cord neural circuit regeneration in zebrafish, an animal model with robust regenerative capacity, to provide insights into the regeneration of the mammalian central nervous system.

Domino reaction of neurovascular unit in neuropathic pain after spinal cord injury

The mechanism of neuropathic pain after spinal cord injury is complex, and the communication between neurons, glia, and blood vessels in neurovascular units significantly affects the occurrence and development of neuropathic pain. After spinal cord injury, a domino chain reaction occurs in the neuron-glia-vessel, which affects the permeability of the blood-spinal cord barrier and jointly promotes the development of neuroinflammation. This article discusses the signal transduction between neuro-glial-endothelial networks from a multidimensional point of view and reviews its role in neuropathic pain after spinal cord injury.

Hematogenous Macrophages Contribute to Fibrotic Scar Formation After Optic Nerve Crush

Although glial scar formation has been extensively studied after optic nerve injury, the existence and characteristics of traumatic optic nerve fibrotic scar formation have not been previously characterized. Recent evidence suggests infiltrating macrophages are involved in pathological processes after optic nerve crush (ONC), but their role in fibrotic scar formation is unknown. Using wild-type and transgenic mouse models with optic nerve crush injury, we show that macrophages infiltrate and associate with fibroblasts in the traumatic optic nerve lesion fibrotic scar. We dissected the role of hematogenous and resident macrophages, labeled with Dil liposomes intravenously administered, and observed that hematogenous macrophages (Dil+ cells) specifically accumulate in the center of traumatic fibrotic scar while Iba-1+ cells reside predominantly at the margins of optic nerve fibrotic scar. Depletion of hematogenous macrophages results in reduced fibroblast density and decreased extracellular matrix deposition within the fibrotic scar area following ONC. However, retinal ganglion cell degeneration and function loss after optic nerve crush remain unaffected after hematogenous macrophage depletion. We present new and previously not characterized evidence that hematogenous macrophages are selectively recruited into the fibrotic core of the optic nerve crush site and critical for this fibrotic scar formation.

Macrophage Extracellular Traps Exacerbate Secondary Spinal Cord Injury by Modulating Macrophage/Microglia Polarization via LL37/P2X7R/NF-κB Signaling Pathway

Persistent inflammation in the secondary spinal cord injury (SCI) is an important reason for the failure of nerve repair, which is partly due to the continuous activation of local M1-like macrophage/microglia. It is reported that extracellular trap (ET) has been a new way of cell death, which can be released by macrophages and named macrophage extracellular trap (Met). Furthermore, it exists widely in the pathophysiological process of many diseases, but it has been rarely studied in the field of SCI. In this study, we constructed a spinal cord contusion model and assessed the function outcome of SCI rats. We used immunofluorescence, flow cytometry, and transmission electron microscope (TEM) to demonstrate the existence of Mets. Besides, some related experiments had also been employed to explore the relationship between Mets and M1 polarization of macrophage/microglia. We also performed Co-IP and Western blotting to reveal a new extracellular proinflammatory signal pathway. Finally, we made a linear regression analysis between the concentrations of specific markers of Mets in human serum and ASIA scores. Briefly, our results suggested that macrophages infiltrated in SCI area could induce macrophage/microglia to differentiate into M1-like cells by releasing Mets, which may be achieved partly through LL37-P2X37-NF-κB signal pathway. However, limiting Mets could effectively inhibit M1 polarization and promote function recovery. In addition, the concentrations of Met related proteins in human serum showed high correlation with ASIA scores and could be applied to reflect the severity of SCI. In conclusion, Mets may be a new target for SCI therapy and a promising index for SCI assessment.

CNS and CNS diseases in relation to their immune system

The central nervous system is the most important nervous system in vertebrates, which is responsible for transmitting information to the peripheral nervous system and controlling the body's activities. It mainly consists of the brain and spinal cord, which contains rich of neurons, the precision of the neural structures susceptible to damage from the outside world and from the internal factors of inflammation infection, leading to a series of central nervous system diseases, such as traumatic brain injury, nerve inflammation, etc., these diseases may cause irreversible damage on the central nervous or lead to subsequent chronic lesions. After disease or injury, the immune system of the central nervous system will play a role, releasing cytokines to recruit immune cells to enter, and the immune cells will differentiate according to the location and degree of the lesion, and become specific immune cells with different functions, recognize and phagocytose inflammatory factors, and repair the damaged neural structure. However, if the response of these immune cells is not suppressed, the overexpression of some genes can cause further damage to the central nervous system. There is a need to understand the molecular mechanisms by which these immune cells work, and this information may lead to immunotherapies that target certain diseases and avoid over-activation of immune cells. In this review, we summarized several immune cells that mainly play a role in the central nervous system and their roles, and also explained the response process of the immune system in the process of some common neurological diseases, which may provide new insights into the central nervous system.

Stem Cell Strategies in Promoting Neuronal Regeneration after Spinal Cord Injury: A Systematic Review

Spinal cord injury (SCI) is a devastating condition with a significant medical and socioeconomic impact. To date, no effective treatment is available that can enable neuronal regeneration and recovery of function at the damaged level. This is thought to be due to scar formation, axonal degeneration and a strong inflammatory response inducing a loss of neurons followed by a cascade of events that leads to further spinal cord damage. Many experimental studies demonstrate the therapeutic effect of stem cells in SCI due to their ability to differentiate into neuronal cells and release neurotrophic factors. Therefore, it appears to be a valid strategy to use in the field of regenerative medicine. This review aims to provide an up-to-date summary of the current research status, challenges, and future directions for stem cell therapy in SCI models, providing an overview of this constantly evolving and promising field.

Consequences and mechanisms of myelin debris uptake and processing by cells in the central nervous system

Central nervous system (CNS) disorders and trauma involving changes to the neuronal myelin sheath have long been a topic of great interest. One common pathological change in these diseases is the generation of myelin debris resulting from the breakdown of the myelin sheath. Myelin debris contains many inflammatory and neurotoxic factors that inhibit remyelination and make its clearance a prerequisite for healing in CNS disorders. Many professional and semiprofessional phagocytes participate in the clearance of myelin debris in the CNS. These cells use various mechanisms for the uptake of myelin debris, and each cell type produces its own unique set of pathologic consequences resulting from the debris uptake. Examining these cells' phagocytosis of myelin debris will contribute to a more complete understanding of CNS disease pathogenesis and help us conceptualize how the necessary clearance of myelin debris must be balanced with the detrimental consequences brought about by its clearance.

Genetic deletion of the glucocorticoid receptor in Cx3cr1+ myeloid cells is neuroprotective and improves motor recovery after spinal cord injury

Glucocorticoid receptors (GRs), part of the nuclear receptor superfamily of transcription factors (TFs), are ubiquitously expressed in all cell types and regulate cellular responses to glucocorticoids (e.g., cortisol in humans; corticosterone in rodents). In myeloid cells, glucocorticoids binding to GRs can enhance or repress gene transcription, thereby imparting distinct and context-dependent functions in macrophages at sites of inflammation. In experimental models and in humans, glucocorticoids are widely used as anti-inflammatory treatments to promote recovery of function after SCI. Thus, we predicted that deleting GR in mouse myeloid lineage cells (i.e., microglia and monocyte-derived macrophages) would enhance inflammation at the site of injury and worsen functional recovery after traumatic spinal cord injury (SCI). Contrary to our prediction, the intraspinal macrophage response to a moderate (75 kdyne) spinal contusion SCI was reduced in Cx3cr1-Cre;GRf/f conditional knockout mice (with GR specifically deleted in myeloid cells). This phenotype was associated with improvements in hindlimb motor recovery, myelin sparing, axon sparing/regeneration, and microvascular protection/plasticity relative to SCI mice with normal myeloid cell GR expression. Further analysis revealed that macrophage GR deletion impaired lipid and myelin phagocytosis and foamy macrophage formation. Together, these data reveal endogenous GR signaling as a key pathway that normally inhibits mechanisms of macrophage-mediated repair after SCI.

Bioinformatics analysis identified apolipoprotein E as a hub gene regulating neuroinflammation in macrophages and microglia following spinal cord injury

Macrophages and microglia play important roles in chronic neuroinflammation following spinal cord injury (SCI). Although macrophages and microglia have similar functions, their phagocytic and homeostatic abilities differ. It is difficult to distinguish between these two populations in vivo, but single-cell analysis can improve our understanding of their identity and heterogeneity. We conducted bioinformatics analysis of the single-cell RNA sequencing dataset GSE159638, identifying apolipoprotein E (APOE) as a hub gene in both macrophages and microglia in the subacute and chronic phases of SCI. We then validated these transcriptomic changes in a mouse model of cervical spinal cord hemi-contusion and observed myelin uptake, lipid droplets, and lysosome accumulation in macrophages and microglia following SCI. Finally, we observed that knocking out APOE aggravated neurological dysfunction, increased neuroinflammation, and exacerbated the loss of white matter. Targeting APOE and the related cholesterol efflux represents a promising strategy for reducing neuroinflammation and promoting recovery following SCI.

Macrophages Treated with VEGF and PDGF Exert Paracrine Effects on Olfactory Ensheathing Cell Function

Glial cell transplantation using olfactory ensheathing cells (OECs) holds a promising approach for treating spinal cord injury (SCI). However, integration of OECs into the hostile acute secondary injury site requires interaction and response to macrophages. Immunomodulation of macrophages to reduce their impact on OECs may improve the functionality of OECs. Vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF), known for their immunomodulatory and neuroprotective functions, have provided improved outcomes in SCI animal models. Thus, VEGF and PDGF modulation of the SCI microenvironment may be beneficial for OEC transplantation. In this in vitro study, the effect of VEGF and PDGF on macrophages in an inflammatory condition was tested. Combined VEGF + PDGF reduced translocation nuclear factor kappa B p65 in macrophages without altering pro-inflammatory cytokines. Further, the ability of OECs to phagocytose myelin debris was assessed using macrophage-conditioned medium. Conditioned medium from macrophages incubated with PDGF and combined VEGF + PDGF in inflammatory conditions promoted phagocytosis by OECs. The growth factor treated conditioned media also modulated the expression of genes associated with nerve repair and myelin expression in OECs. Overall, these results suggest that the use of growth factors together with OEC transplantation may be beneficial in SCI therapy.

Fibrotic Scar in CNS Injuries: From the Cellular Origins of Fibroblasts to the Molecular Processes of Fibrotic Scar Formation

Central nervous system (CNS) trauma activates a persistent repair response that leads to fibrotic scar formation within the lesion. This scarring is similar to other organ fibrosis in many ways; however, the unique features of the CNS differentiate it from other organs. In this review, we discuss fibrotic scar formation in CNS trauma, including the cellular origins of fibroblasts, the mechanism of fibrotic scar formation following an injury, as well as the implication of the fibrotic scar in CNS tissue remodeling and regeneration. While discussing the shared features of CNS fibrotic scar and fibrosis outside the CNS, we highlight their differences and discuss therapeutic targets that may enhance regeneration in the CNS.

Microglia coordinate cellular interactions during spinal cord repair in mice

Traumatic spinal cord injury (SCI) triggers a neuro-inflammatory response dominated by tissue-resident microglia and monocyte derived macrophages (MDMs). Since activated microglia and MDMs are morphologically identical and express similar phenotypic markers in vivo, identifying injury responses specifically coordinated by microglia has historically been challenging. Here, we pharmacologically depleted microglia and use anatomical, histopathological, tract tracing, bulk and single cell RNA sequencing to reveal the cellular and molecular responses to SCI controlled by microglia. We show that microglia are vital for SCI recovery and coordinate injury responses in CNS-resident glia and infiltrating leukocytes. Depleting microglia exacerbates tissue damage and worsens functional recovery. Conversely, restoring select microglia-dependent signaling axes, identified through sequencing data, in microglia depleted mice prevents secondary damage and promotes recovery. Additional bioinformatics analyses reveal that optimal repair after SCI might be achieved by co-opting key ligand-receptor interactions between microglia, astrocytes and MDMs.

Mesenchymal Stem Cell-Derived Exosomal MiRNAs Promote M2 Macrophages Polarization: Therapeutic Opportunities for Spinal Cord Injury

Spinal cord injury (SCI) is an enormous public health concern affecting approximately 250,000–500,000 people worldwide each year. It is mostly irreversible considering the limitations of currently available treatments, and its prevention and management have been the prime focus of many studies. Mesenchymal stem cell (MSC) transplantation is one of the most promising treatments for SCI. The role of MSCs in SCI has been studied extensively, and MSCs have been shown to have many limitations. Moreover, the therapeutic effects of MSCs are more likely related to paracrine effects. In SCIs, macrophages from peripheral sources differentiate into M1 macrophages, promoting inflammation and aggravating neuronal damage; however, studies have shown that MSC-derived exosomes can induce the polarization of macrophages from the M1 to the M2 phenotype, thereby promoting nerve function recovery in patients with SCI. In this review, we discussed the research progress of MSC-derived exosomal miRNAs in promoting M2 macrophage differentiation in the SCI, and introduced some exosomal miRNAs that can regulate the differentiation of M2 macrophages in non-SCI; it is hoped that the regulatory role of these exosome-derived miRNAs can be confirmed in SCI.

The Role of Microglia/Macrophages Activation and TLR4/NF-κB/MAPK Pathway in Distraction Spinal Cord Injury-Induced Inflammation

Distraction spinal cord injuries (DSCIs) often occur as the neurological complication of distraction forces following the implantation of internal fixation devices during scoliosis correction surgery. However, the underlying mechanism behind these injuries remains unclear. The present study aimed to explore the activation of microglia and macrophages, as well as changes in TLR4-mediated NF-κB and MAPK pathway activity after DSCIs in Bama miniature pigs. Prior to surgical intervention, the pigs were randomly divided into three groups: the sham group, the complete distraction spinal cord injury (CDSCI) group, and the incomplete distraction spinal cord injury (IDSCI) group. After surgery, the Tarlov scale and individual limb motor scale (ILMS) were used to evaluate changes in the pigs’ behavior. All pigs were euthanized 7 days after surgery, and histopathological examinations of the spinal cord tissues were performed. Immunohistochemistry was used to detect Caspase-3 expression in the anterior horn of spinal gray matter tissues. Immunofluorescence staining was utilized to assess the M1/M2 phenotype changes in microglia/macrophages and NF-κB P65 expression in central DSCI lesions, while western blotting was performed to determine the expression of TLR4/NF-κB/MAPK pathway-related proteins. The results of the present study showed that the Tarlov and ILMS scores decreased significantly in the two DSCI groups compared with the sham group. Hematoxylin and eosin (HE) and Nissl staining revealed that the tissue structure and nerve fiber tracts in the distracted spinal cord tissues were destroyed. Both DSCI groups showed the number of survived neurons decreased and the Caspase-3 expression increased. The results of the immunofluorescence staining indicated that the CD16 and CD206 expression in the microglia/macrophages increased. Between the two DSCI groups, the CDSCI group showed increased CD16 and decreased CD206 expression levels. The intensity of the fluorescence of NF-κB P65 was found to be significantly enhanced in pigs with DSCIs. Moreover, western blot results revealed that the expression of TLR4, p-IκBα, NF-κB P65, p-JNK, p-ERK, and p-P38 proteins increased in spinal cord tissues following DSCI. The present study was based on a porcine DSCI model that closely mimicked clinical DSCIs while clarifying DSCI-associated neuroinflammation mechanisms, in turn providing evidence for identifying potential anti-inflammatory targets.

D-4F, an apolipoprotein A-I mimetic, promotes the clearance of myelin debris and the reduction of foamy macrophages after spinal cord injury

After spinal cord injury (SCI), a large number of blood-derived macrophages infiltrate the lesion site and phagocytose myelin debris to become foamy macrophages, which leads to chronic inflammation. The drug D-4F, an apolipoprotein A-I peptidomimetic made of D-amino acids, has been reported to promote the lipid metabolism of foamy macrophages in atherosclerosis. However, the role and mechanism of D-4F in SCI are still unclear. In this study, we found that D-4F can promote the removal of myelin debris, reduce the formation of foamy macrophages in the lesion core and promote neuroprotection and recovery of motor function after SCI. These beneficial functions of D-4F may be related to its ability to upregulate the expression of ATP-binding cassette transporter A1 (ABCA1), the main transporter that mediates lipid efflux in foamy macrophages because inhibiting the activity of ABCA1 can reverse the effect of D-4F in vitro. In conclusion, D-4F may be a promising candidate for treating SCI by promoting the clearance of myelin debris by foamy macrophages via the ABCA1 pathway.

Single-cell transcriptome analysis reveals the immune heterogeneity and the repopulation of microglia by Hif1α in mice after spinal cord injury

Neuroinflammation is regarded as a vital pathological process in spinal cord injury (SCI), which removes damaged tissue, secretes cytokines, and facilitates regeneration. Repopulation of microglia has been shown to favor recovery from SCI. However, the origin and regulatory factors of microglia repopulation after SCI remain unknown. Here, we used single-cell RNA sequencing to portray the dynamic transcriptional landscape of immune cells during the early and late phases of SCI in mice. B cells and migDCs, located in the meninges under physiological conditions, are involved in immune surveillance. Microglia quickly reduced, and peripheral myeloid cells infiltrated three days-post-injury (dpi). At 14 dpi, microglia repopulated, myeloid cells were reduced, and lymphocytes infiltrated. Importantly, genetic lineage tracing of nestin⁺ and Cx3cr1⁺ cells in vivo showed that the repopulation of microglia was derived from residual microglia after SCI. We found that residual microglia regress to a developmental growth state in the early stages after SCI. Hif1α promotes microglial proliferation. Conditional ablation of Hif1α in microglia causes larger lesion sizes, fewer axon fibers, and impaired functional recovery in the late stages after SCI. Our results mapped the immune heterogeneity in SCI and raised the possibility that targeting Hif1α may help in axon regeneration and functional recovery after SCI.

Macrophage-based delivery of interleukin-13 improves functional and histopathological outcomes following spinal cord injury

BACKGROUND

Spinal cord injury (SCI) elicits a robust neuroinflammatory reaction which, in turn, exacerbates the initial mechanical damage. Pivotal players orchestrating this response are macrophages (Mφs) and microglia. After SCI, the inflammatory environment is dominated by pro-inflammatory Mφs/microglia, which contribute to secondary cell death and prevent regeneration. Therefore, reprogramming Mφ/microglia towards a more anti-inflammatory and potentially neuroprotective phenotype has gained substantial therapeutic interest in recent years. Interleukin-13 (IL-13) is a potent inducer of such an anti-inflammatory phenotype. In this study, we used genetically modified Mφs as carriers to continuously secrete IL-13 (IL-13 Mφs) at the lesion site.

METHODS

Mφs were genetically modified to secrete IL-13 (IL-13 Mφs) and were phenotypically characterized using qPCR, western blot, and ELISA. To analyze the therapeutic potential, the IL-13 Mφs were intraspinally injected at the perilesional area after hemisection SCI in female mice. Functional recovery and histopathological improvements were evaluated using the Basso Mouse Scale score and immunohistochemistry. Neuroprotective effects of IL-13 were investigated using different cell viability assays in murine and human neuroblastoma cell lines, human neurospheroids, as well as murine organotypic brain slice cultures.

RESULTS

In contrast to Mφs prestimulated with recombinant IL-13, perilesional transplantation of IL-13 Mφs promoted functional recovery following SCI in mice. This improvement was accompanied by reduced lesion size and demyelinated area. The local anti-inflammatory shift induced by IL-13 Mφs resulted in reduced neuronal death and fewer contacts between dystrophic axons and Mφs/microglia, suggesting suppression of axonal dieback. Using IL-4Rα-deficient mice, we show that IL-13 signaling is required for these beneficial effects. Whereas direct neuroprotective effects of IL-13 on murine and human neuroblastoma cell lines or human neurospheroid cultures were absent, IL-13 rescued murine organotypic brain slices from cell death, probably by indirectly modulating the Mφ/microglia responses.

CONCLUSIONS

Collectively, our data suggest that the IL-13-induced anti-inflammatory Mφ/microglia phenotype can preserve neuronal tissue and ameliorate axonal dieback, thereby promoting recovery after SCI.

New Insight in HDACs: Potential Therapeutic Targets for the Treatment of Atherosclerosis

Atherosclerosis (AS) features include progressive hardening and reduced elasticity of arteries. AS is the leading cause of morbidity and mortality. An increasing amount of evidence showed that epigenetic modifications on genes serve are a main cause of several diseases, including AS. Histone deacetylases (HDACs) promote the deacetylation at lysine residues, thereby condensing the chromatin structures and further inhibiting the transcription of downstream genes. HDACs widely affect various physiological and pathological processes through transcriptional regulation or deacetylation of other non-histone proteins. In recent years, the role of HDACs in vascular systems has been revealed, and their effects on atherosclerosis have been widely reported. In this review, we discuss the members of HDACs in vascular systems, determine the diverse roles of HDACs in AS, and reveal the effects of HDAC inhibitors on AS progression. We provide new insights into the potential of HDAC inhibitors as drugs for AS treatment.

Mitochondrial function in spinal cord injury and regeneration

Many people around the world suffer from some form of paralysis caused by spinal cord injury (SCI), which has an impact on quality and life expectancy. The spinal cord is part of the central nervous system (CNS), which in mammals is unable to regenerate, and to date, there is a lack of full functional recovery therapies for SCI. These injuries start with a rapid and mechanical insult, followed by a secondary phase leading progressively to greater damage. This secondary phase can be potentially modifiable through targeted therapies. The growing literature, derived from mammalian and regenerative model studies, supports a leading role for mitochondria in every cellular response after SCI: mitochondrial dysfunction is the common event of different triggers leading to cell death, cellular metabolism regulates the immune response, mitochondrial number and localization correlate with axon regenerative capacity, while mitochondrial abundance and substrate utilization regulate neural stem progenitor cells self-renewal and differentiation. Herein, we present a comprehensive review of the cellular responses during the secondary phase of SCI, the mitochondrial contribution to each of them, as well as evidence of mitochondrial involvement in spinal cord regeneration, suggesting that a more in-depth study of mitochondrial function and regulation is needed to identify potential targets for SCI therapeutic intervention.

Long-term in vivo imaging of mouse spinal cord through an optically cleared intervertebral window

The spinal cord accounts for the main communication pathway between the brain and the peripheral nervous system. Spinal cord injury is a devastating and largely irreversible neurological trauma, and can result in lifelong disability and paralysis with no available cure. In vivo spinal cord imaging in mouse models without introducing immunological artifacts is critical to understand spinal cord pathology and discover effective treatments. We developed a minimally invasive intervertebral window by retaining the ligamentum flavum to protect the underlying spinal cord. By introducing an optical clearing method, we achieve repeated two-photon fluorescence and stimulated Raman scattering imaging at subcellular resolution with up to 15 imaging sessions over 6-167 days and observe no inflammatory response. Using this optically cleared intervertebral window, we study neuron-glia dynamics following laser axotomy and observe strengthened contact of microglia with the nodes of Ranvier during axonal degeneration. By enabling long-term, repetitive, stable, high-resolution and inflammation-free imaging of mouse spinal cord, our method provides a reliable platform in the research aiming at interpretation of spinal cord physiology and pathology.

Vagus Nerve Stimulation Reduces Neuroinflammation Through Microglia Polarization Regulation to Improve Functional Recovery After Spinal Cord Injury

Background

Spinal cord injury (SCI) is a devastating disease that lacks effective treatment. Interestingly, recent studies indicated that vagus nerve stimulation (VNS), neuromodulation that is widely used in a variety of central nervous system (CNS) diseases, improved motor function recovery after SCI. But the exact underlying mechanism of how VNS ameliorates SCI is unclear. This study aimed to confirm the efficacy and further explore the potential therapeutic mechanism of VNS in SCI.

Method

A T10 spinal cord compression model was established in adult female Sprague-Dawley rats. Then the stimulation electrode was placed in the left cervical vagus nerve (forming Sham-VNS, VNS, and VNS-MLA groups). Basso-Beattie-Bresnahan (BBB) behavioral scores and Motor evoked potentials (MEPs) analysis were used to detect motor function. A combination of histological and molecular methods was used to clarify the relevant mechanism.

Results

Compared with the Sham-VNS group, the VNS group exhibited better functional recovery, reduced scar formation (both glial and fibrotic scars), tissue damage, and dark neurons, but these beneficial effects of VNS were diminished after alpha 7 nicotinic acetylcholine receptor (α7nAchR) blockade. Specifically, VNS inhibited the pro-inflammatory factors TNF-α, IL-1β, and IL-6 and increased the expression of the anti-inflammatory factors IL-10. Furthermore, we found that VNS promotes the shift of M1-polarized Iba-1+/CD86+ microglia to M2-polarized Iba-1+/CD206+ microglia via upregulating α7nAchR to alleviate neuroinflammation after SCI.

Conclusion

Our results demonstrated that VNS promotes microglial M2 polarization through upregulating α7nAChR to reduce neuroinflammation, thus improving motor function recovery after SCI. These findings indicate VNS might be a promising neuromodulation strategy for SCI.

Foamy macrophages potentially inhibit tuberculous wound healing by inhibiting the TLRs/NF-κB signalling pathway

To characterise the distribution, classification, and quantity of foamy macrophages (FMs) in tuberculous wound tissue and the relationship between FM and delayed healing of tuberculous wounds. Morphological studies were performed to explore the distribution of FM and Mycobacterium tuberculosis (Mtb) in tuberculous wounds, with acute and chronic wounds included for comparison. Phorbol-12-myristate-13-acetate stimulation-differentiated THP-1 cells were treated with Mtb to induce their differentiation into FM with oxidised low-density lipoprotein treatment serving as a control. Relative cytokine levels were determined by quantitative PCR and Western blotting. Varied co-culture combinations of Mtb, THP-1, FM, and fibroblasts were performed, and proliferation, migration, ability to contract collagen gel, and protein levels of the chemokines in the supernatants of the fibroblasts were assessed. The differentially expressed genes in human skin fibroblasts (HSFs) after co-culture with or without FM were identified using microarray. Many FM were found in the tissues of tuberculous wounds. The FM that did not engulf Mtb (NM-FM) were mainly distributed in tissues surrounding tuberculous wounds, whereas the FM that engulfed Mtb (M-FM) were dominantly located within granulomatous tissues. Co-culture experiments showed that, with the Mtb co-culture, the portions of NM-FM in the total FM grew over time. The migration, proliferation, chemokine secretion, and the ability of fibroblasts to contract collagen gel were inhibited when co-cultured with Mtb, FM, or a combination of the two. Further investigation showed that the TLRs/NF-κB signalling pathway is involved in fibroblast function under the stimulation of FM. TLRs and NF-κB agonists could reverse the phenotypic changes in HSFs after co-culture with FM. The tuberculous wound microenvironment composed of Mtb and FM may affect wound healing by inhibiting the functions of fibroblasts. FM potentially inhibit fibroblasts' function by inhibiting the TLRs/NF-κB signalling pathway in tuberculous wounds.

Photobiomodulation Attenuates Neurotoxic Polarization of Macrophages by Inhibiting the Notch1-HIF-1α/NF-κB Signalling Pathway in Mice With Spinal Cord Injury

Spinal cord injury (SCI) is a catastrophic disease with a complex pathogenesis that includes inflammation, oxidative stress, and glial scar formation. Macrophages are the main mediators of the inflammatory response and are distributed in the epicentre of the SCI. Macrophages have neurotoxic and neuroprotective phenotypes (also known as classically and alternatively activated macrophages or M1 and M2 macrophages) that are associated with pro- or anti- inflammatory gene expression. Our previous study demonstrated that photobiomodulation (PBM) alters the polarization state of macrophages in the SCI region towards the M2 phenotype and promotes the recovery of motor function in rats with SCI. However, the mechanism by which PBM promotes SCI repair remains largely undefined. This study is based on the replacement of conventional percutaneous irradiation with implantable biofibre optic in vivo irradiation. The aim was to further investigate the effects of PBM on SCI in mice under new irradiation patterns and its potential mechanisms of action. PBM was administered to male mice with clamped SCI for four consecutive weeks and significantly promoted the recovery of motor function in mice. Analysis of the macrophage phenotypes in the epicentre of the SCI in mice showed that PBM mainly inhibited the neurotoxic activation of macrophages in the SCI area and reduced the secretion of inflammatory factors such as IL-1α and IL-6; PBM had no effect on M2 macrophages. Immediately afterwards, we constructed in vitro models of the inflammatory polarization of macrophages and PBM intervention. We found that PBM attenuated the neurotoxicity of M1 macrophages on VSC 4.1 motor neurons and dorsal root ganglion (DRG) neurons. The effects of PBM on neurotoxic macrophages and the possible mechanisms of action were analysed using RNA sequencing (RNA-seq), which confirmed that the main role of PBM was to modulate the inflammatory response and immune system processes. Analysis of the differentially expressed genes (DEGs) associated with the inflammatory response showed that PBM had the most significant regulatory effects on genes such as interleukin (IL)-1α, IL-6, cyclooxygenase-2 (COX-2), and inducible nitric oxide synthase (iNOS) and had obvious inhibitory effects on inflammation-related Notch1 and hypoxia-inducible factor-1α (HIF-1α) pathway genes. RNA-seq analysis of the effect of PBM on gene expression in resting-state macrophages and M2 macrophages did not show significant differences (data not shown). In conclusion, PBM promoted better motor recovery after SCI in mice by inhibiting the neurotoxic polarization of macrophages and the release of inflammatory mediators by acting on the Notch1-HIF-1α/NF-κB Signalling Pathway.

Fatty acid-binding protein 4 (FABP4) inhibition promotes locomotor and autonomic recovery in rats following spinal cord injury

The inflammatory response associated with traumatic spinal cord injury (SCI) contributes to locomotor and sensory impairments. Pro-inflammatory (M1) macrophages/microglia (MφMG) are the major cellular players in this response as they promote chronic inflammation resulting in injury expansion and tissue damage. Fatty Acid-Binding Protein 4 (FABP4) promotes M1 MφMG differentiation; however, it is unknown if FABP4 also plays a role in the etiology of SCI. The present study investigates whether FABP4's gene expression influences functional recovery following SCI. Analysis of qPCR data shows a robust induction of FABP4 mRNA (>100 fold) in rats subjected to a T9-T10 contusion injury compared to control. Western blot experiments reveal significant upregulation of FABP4 protein at the injury epicenter, and immunofluorescence analysis identifies this upregulation occurs in CD11b+ MφMG. Furthermore, upregulation of FABP4 gene expression correlates with PPARγ downregulation, inactivation of Iκβα, and the activation of the NF-κB pathway. Analysis of locomotor recovery using the Basso-Beattie-Bresnahan's (BBB) locomotor scale and the CatWalk gait analysis system shows that injured rats treated with FABP4 inhibitor BMS309403 have significant improvements in locomotion compared to vehicle controls. Additionally, inhibitor-treated rats exhibit enhanced autonomic bladder reflex recovery. Immunofluorescence experiments also show the administration of the FABP4 inhibitor increases the number of CD163+ and Liver Arginase+ M2 MφMG within the epicenter and penumbra of the injured spinal cord 28 dpi. These findings show that FABP4 may significantly exacerbate locomotor and sensory impairments during SCI by modulating macrophage/microglial activity.

Specific labelling of phagosome-derived vesicles in macrophages with a membrane dye delivered with microfabricated microparticles

Phagocytosis performed by a macrophage involves complex membrane trafficking and reorganization among various membranous cellular structures including phagosomes and vesicles derived from the phagosomes known as phagosome-derived vesicles. The present work reports on development of a technique that allows to specifically label the phagosome-derived vesicles in macrophages with a membrane dye. The technique is based on the use of microfabricated microparticles that are made of a thermosensitive nonbiodegradable polymer poly(N-isopropylacrylamide) (PNIPAM) or its derivative and contain a membrane dye 1,1'-dialkyl-3,3,3',3'-tetramethylindodicarbocyanine (DiI). The microparticles can be phagocytosed by RAW264.7 macrophages into their phagosomes, resulting in formation of intracellular DiI-positive vesicles derived from the phagosomes. The DiI-positive vesicles are motile and acidic; can be stained by fluorescently labelled dextran added in the culture medium; and can accumulate around new phagosomes, indicating that they possess properties of lysosomes. This technique is also applicable to another membrane dye 3,3'-dioctadecyloxacarbocyanine (DiO) and holds great potential to be useful for advancing our understanding of phagocytosis. STATEMENT OF SIGNIFICANCE: Phagocytosis performed by macrophages is a cellular process of great importance to various applications of biomaterials such as drug delivery and medical implantation. This work reports on a technique for characterizing phagocytosis based on the use of poly(N-isopropylacrylamide), which is a major biomaterial with numerous applications. This technique is the first of its kind and has generated an original finding about phagocytosis. In addition to drug delivery and medical implantation, phagocytosis plays critical roles in diseases, injuries and vaccination. This work could thus attract immediate and widespread interests in the field of biomaterials science and engineering.

Myelin and non-myelin debris contribute to foamy macrophage formation after spinal cord injury

Tissue damage after spinal cord injury (SCI) elicits a robust inflammatory cascade that fails to resolve in a timely manner, resulting in impaired wound healing and cellular regeneration. This inflammatory response is partly mediated by infiltrating immune cells, including macrophages. As professional phagocytes, macrophages initially play an important role in debris clearance at the injury site, which would be necessary for proper tissue regeneration. After SCI, most macrophages become filled with lipid droplets due to excessive uptake of lipid debris, assuming a "foamy" phenotype that is associated with a proinflammatory state. Myelin has been assumed to be the main source of lipid that induces foamy macrophage formation after injury given its abundance in the spinal cord. This assumption has led to the widespread use of purified myelin treatment to model foamy macrophage formation in vitro. However, the assumption that myelin is necessary for foamy macrophage formation remains untested. To this end, we developed a novel foamy macrophage assay utilizing total spinal cord homogenate to include all sources of lipid present at the injury site. Using the myelin basic protein knockout (MBP KO, i.e., Shiverer) mice that lack myelin, we investigated lipid accumulation in foamy macrophages. Primary macrophages treated with myelin-deficient spinal cord homogenate still formed large lipid droplets typically observed in foamy macrophages, although to a lesser degree than cells treated with normal homogenate. Similarly, MBP KO mice subjected to contusive spinal cord injury also formed foamy macrophages that exhibited reduced lipid content and associated with improved histological outcomes and reduced immune cell infiltration. Therefore, the absence of myelin does not preclude foamy macrophage formation, indicating that myelin is not the only major source of lipid that contributes this pathology, even though myelin may alter certain aspects of its inflammatory profile.

M1 macrophages impair tight junctions between endothelial cells after spinal cord injury

After spinal cord injury (SCI), endogenous angiogenesis occurs in the injury core, unexpectedly accompanied by continuous leakage of the blood-spinal cord barrier (BSCB), which may be caused by destruction of the tight junctions (TJs) between vascular endothelial cells-an important structure of the BSCB. Blood-derived macrophages infiltrate into the spinal cord, aggregate to the injury core and then polarize toward M1/M2 phenotypes after SCI. However, the effect of macrophages with different polarizations on the TJs between vascular endothelial cells remains unclear. Here, we demonstrated that from 7 days postinjury (dpi) to 28 dpi, accompanied by the aggregation of macrophages, the expression of claudin-5 (CLN-5) and zonula occludens-1 (ZO-1) in vascular endothelial cells in the injury core was significantly decreased in comparison to that in normal spinal cord tissue and in the penumbra. Moreover, the leakage of the BSCB was severe in the injury core, as demonstrated by FITC-dextran perfusion. Notably, our study demonstrated that depletion of macrophages facilitated the restoration of TJs between vascular endothelial cells and decreased the leakage of BSCB in the injury core after SCI. Furthermore, we confirmed that the endothelial TJs could be impaired by M1 macrophages through secreting IL-6 in vitro, leading to an increased permeability of endothelial cells, but it was not significantly affected by M0 and M2 macrophages. These results indicated that the TJs between vascular endothelial cells were impaired by M1 macrophages in the injury core, potentially resulting in continuous leakage of the BSCB after SCI. Preventing M1 polarization of macrophages or blocking IL-6 in the injury core may promote restoration of the BSCB, thus accelerating functional recovery after SCI.

Immune and Metabolic Biomarkers in a Rodent Model of Spinal Cord Contusion

STUDY DESIGN

Basic science animal research study.

OBJECTIVES

Using T10 spinal contused rats, we sought to identify molecular and circulating, metabolic and immune biomarkers during the subchronic and chronic recovery periods that may inform us concerning neurorehabilitation.

METHODS

Gene expression of the cord and ELISA were performed in 28 and 100 days in T10 injured rats and compared to sham-injured rats. Hundred-day injured rats were placed on either a low-fat or high-fat diet following the recovery phase. Linear regression analysis was performed between markers and locomotor score, body weight, body composition, and blood cholesterol and triglycerides.

RESULTS

Gene expression in the thoracic cord for complement marker, C1QC, dendritic cell marker, ITGAX, and cholesterol biosynthesis genes, FDFT1, HMCGR, LDLR, and SREBP1, were significantly associated with BBB score, body weight, composition, and other metabolic parameters. Circulating levels of these proteins, however, did not vary by injury or predict the level of locomotor recovery.

CONCLUSIONS

Identification of reliable circulating biomarkers that are durable and based on level of spinal injury are complicated by immune and metabolic comorbidities. Continued work is necessary to identify stable markers of disease progression.

Immune response after central nervous system injury

Traumatic injuries of the central nervous system (CNS) affect millions of people worldwide, and they can lead to severely damaging consequences such as permanent disability and paralysis. Multiple factors can obstruct recovery after CNS injury. One of the most significant is the progressive neuronal death that follows the initial mechanical impact, leading to the loss of undamaged cells via a process termed secondary neurodegeneration. Efforts to define treatments that limit the spread of damage, while important, have been largely ineffectual owing to gaps in the mechanistic understanding that underlies the persisting neuronal cell death. Inflammation, with its influx of immune cells that occurs shortly after injury, has been associated with secondary neurodegeneration. However, the role of the immune system after CNS injury is far more complex. Studies have indicated that the immune response after CNS injury is detrimental, owing to immune cell-produced factors (e.g., pro-inflammatory cytokines, free radicals, neurotoxic glutamate) that worsen tissue damage. Our lab and others have also demonstrated the beneficial immune response that occurs after CNS injury, with the release of growth factors such as brain-derived growth factor (BDNF) and interleukin (IL-10) and the clearance of apoptotic and myelin debris by immune cells^1-4. In this review, we first discuss the multifaceted roles of the immune system after CNS injury. We then speculate on how advancements in single-cell RNA technologies can dramatically change our understanding of the immune response, how the spinal cord meninges serve as an important site for hosting immunological processes critical for recovery, and how the origin of peripherally recruited immune cells impacts their function in the injured CNS.

Polaryzacja mikrogleju i makrofagów w wybranych chorobach degeneracyjnych i zapalnych układu nerwowego

Makrofagi to komórki efektorowe układu odpornościowego zdolne do polaryzacji, czyli zmiany fenotypu powiązanej ze zmianą aktywności. Można wyróżnić: polaryzację klasyczną (M1), która służy obronie przed patogenami, a makrofagi M1 mają aktywność ogólnie prozapalną, oraz polaryzację alternatywną (M2), która sprzyja wygaszaniu stanu zapalnego i regeneracji tkanki. Makrofagi zasiedlają niemal cały organizm, więc zjawisko ich polaryzacji ma wpływ na wiele procesów zachodzących w różnych tkankach. W układzie nerwowym reprezentacją osiadłych makrofagów jest mikroglej. Jednak w wielu sytuacjach patologicznych w mózgu pojawiają się także makrofagi rekrutowane z monocytów krążących we krwi. Choroby neurodegeneracyjne, urazy i choroby autoimmunologiczne są związane z reakcją układu odpornościowego, która może mieć istotny wpływ na dalszy przebieg choroby i na tempo regeneracji tkanki. Polaryzacja makrofagów ma w związku z tym znaczenie w chorobach centralnego układu nerwowego. Aktywność komórek M1 i M2 może bowiem różnie wpływać na przeżywalność neuronów i oligodendrocytów, na wzrost aksonów, na proces demielinizacji czy na szczelność bariery krew–mózg. Wynika to z różnic między fenotypami w wytwarzaniu reaktywnych form tlenu i tlenku azotu, wydzielaniu cytokin i czynników wzrostu, bezpośrednich oddziaływaniach na sąsiednie komórki i zdolnościach do fagocytozy. W artykule omówiono to zagadnienie w: udarze mózgu, urazie rdzenia kręgowego, chorobie Alzheimera, stwardnieniu zanikowym bocznym i stwardnieniu rozsianym. W wielu spośród tych patologii obserwuje się gradient czasowy lub przestrzenny rozmieszczenia w tkance poszczególnych fenotypów mikrogleju i/lub makrofagów. Wydaje się zatem, że zmiany polaryzacji makrofagów mogą potencjalnie sprzyjać regeneracji tkanki lub hamować rozwój chorób neurodegeneracyjnych.

Citrullinated myelin induces microglial TNFα and inhibits endogenous repair in the cuprizone model of demyelination

BACKGROUND

Microglia are the primary phagocytes of the central nervous system and are responsible for removing damaged myelin following demyelination. Previous investigations exploring the consequences of myelin phagocytosis on microglial activation overlooked the biochemical modifications present on myelin debris. Such modifications, including citrullination, are increased within the inflammatory environment of multiple sclerosis lesions.

METHODS

Mouse cortical myelin isolated by ultracentrifugation was citrullinated ex vivo by incubation with the calcium-dependent peptidyl arginine deiminase PAD2. Demyelination was induced by 6 weeks of cuprizone (0.3%) treatment and spontaneous repair was initiated by reversion to normal chow. Citrullinated or unmodified myelin was injected into the primary motor cortex above the cingulum bundle at the time of reversion to normal chow and the consequent impact on remyelination was assessed by measuring the surface area of myelin basic protein-positive fibers in the cortex 3 weeks later. Microglial responses to myelin were characterized by measuring cytokine release, assessing flow cytometric markers of microglial activation, and RNAseq profiling of transcriptional changes.

RESULTS

Citrullinated myelin induced a unique microglial response marked by increased tumor necrosis factor α (TNFα) production both in vitro and in vivo. This response was not induced by unmodified myelin. Injection of citrullinated myelin but not unmodified myelin into the cortex of cuprizone-demyelinated mice significantly inhibited spontaneous remyelination. Antibody-mediated neutralization of TNFα blocked this effect and restored remyelination to normal levels.

CONCLUSIONS

These findings highlight the role of post-translation modifications such as citrullination in the determination of microglial activation in response to myelin during demyelination. The inhibition of endogenous repair induced by citrullinated myelin and the reversal of this effect by neutralization of TNFα may have implications for therapeutic approaches to patients with inflammatory demyelinating disorders.

Spleen tyrosine kinase facilitates neutrophil activation and worsens long-term neurologic deficits after spinal cord injury

Background

Spinal cord injury elicits widespread inflammation that can exacerbate long-term neurologic deficits. Neutrophils are the most abundant immune cell type to invade the spinal cord in the early acute phase after injury, however, their role in secondary pathogenesis and functional recovery remains unclear. We have previously shown that neutrophil functional responses during inflammation are augmented by spleen tyrosine kinase, Syk, a prominent intracellular signaling enzyme. In this study, we evaluated the contribution of Syk towards neutrophil function and long-term neurologic deficits after spinal cord injury.

Methods

Contusive spinal cord injury was performed at thoracic vertebra level 9 in mice with conditional deletion of Syk in neutrophils (Syk^f/fMRP8-Cre). Hindlimb locomotor recovery was evaluated using an open-field test for 35 days following spinal cord injury. Long-term white matter sparing was assessed using eriochrome cyanide staining. Blood-spinal cord barrier disruption was evaluated by immunoblotting. Neutrophil infiltration, activation, effector functions, and cell death were determined by flow cytometry. Cytokine and chemokine expression in neutrophils was assessed using a gene array.

Results

Syk deficiency in neutrophils improved long-term functional recovery after spinal cord injury, but did not promote long-term white matter sparing. Neutrophil activation, cytokine expression, and cell death in the acutely injured spinal cord were attenuated by the genetic loss of Syk while neutrophil infiltration and effector functions were not affected. Acute blood-spinal cord barrier disruption was also unaffected by Syk deficiency in neutrophils.

Conclusions

Syk facilitates specific neutrophil functional responses to spinal cord injury including activation, cytokine expression, and cell death. Long-term neurologic deficits are exacerbated by Syk signaling in neutrophils independent of acute blood-spinal cord barrier disruption and long-term white matter sparing. These findings implicate Syk in pathogenic neutrophil activities that worsen long-term functional recovery after spinal cord injury.

Osteosarcoma exocytosis of soluble LGALS3BP mediates macrophages toward a tumoricidal phenotype

This study aimed to elucidate the interactions between osteosarcoma (OS) and M1 macrophages infiltrated into the tumor microenvironment and to explore the underlying mechanisms whereby M1 macrophages influence the growth of OS, so that novel treatments of OS can be developed. A transwell co-culture system, an indirect conditioned medium culture system and two orthotopic bearing OS models were established to assess for the interplay between M1 macrophages and OS. We found that the co-culture of M1 macrophages with OS cells significantly inhibited the growth of the tumor cells by inducing apoptosis. Furthermore, HSPA1L secreted by M1 macrophages exerted this anti-tumor effect through the IRAK1 and IRAK4 pathways. LGALS3BP secreted by OS cells bound to the ligand LGALS3 on M1 macrophages and thereby induced the secretion of Hspa11 via Akt phosphorylation. In vivo experiments demonstrated that the culture supernatant of OS-stimulated M1 macrophages significantly inhibited the growth of OS, whereas silencing Lgals3bp promoted the progression of OS. In conclusion, OS modifies the phenotype of tumor-associated macrophages (TAMs) and thereby influences the apoptosis of OS cells through soluble factors. The modulation of TAMs may be a promising and effective therapeutic approach in OS.

Hematogenous Macrophages: A New Therapeutic Target for Spinal Cord Injury

Spinal cord injury (SCI) is a devastating disease leading to loss of sensory and motor functions, whose pathological process includes mechanical primary injury and secondary injury. Macrophages play an important role in SCI pathology. According to its origin, it can be divided into resident microglia and peripheral monocyte-derived macrophages (hematogenous Mφ). And it can also be divided into M1-type macrophages and M2-type macrophages on the basis of its functional characteristics. Hematogenous macrophages may contribute to the SCI process through infiltrating, scar forming, phagocytizing debris, and inducing inflammatory response. Although some of the activities of hematogenous macrophages are shown to be beneficial, the role of hematogenous macrophages in SCI remains controversial. In this review, following a brief introduction of hematogenous macrophages, we mainly focus on the function and the controversial role of hematogenous macrophages in SCI, and we propose that hematogenous macrophages may be a new therapeutic target for SCI.

Exosomes-mediated phenotypic switch of macrophages in the immune microenvironment after spinal cord injury

Although accumulating evidence indicated that modulating macrophage polarization could ameliorate the immune microenvironment and facilitate the repair of spinal cord injury (SCI), the underlying mechanism of macrophage phenotypic switch is still poorly understood. Exosomes (Exos), a potential tool of cell-to-cell communication, may play important roles in cell reprogramming. Herein, we investigated the roles of macrophages-derived exosomes played for macrophage polarization in the SCI immune microenvironment. In this study, we found the fraction of M2 macrophages was markedly decreased after SCI. Moreover, the M2 macrophages-derived exosomes could increase the percentage of M2 macrophages, decrease that of M1 macrophages while the M1 macrophages-derived exosomes acted oppositely. According to the results of in silico analyses and molecular experiments verification, this phenotypic switch might be mediated by the exosomal miRNA-mRNA network, in which the miR-23a-3p/PTEN/PI3K/AKT axis might play an important role. In conclusion, our study suggests macrophage polarization that regulated by various interventions might be mediated by their own exosomes at last. Moreover, M2 macrophages-derived exosomes could promote M2 macrophage polarization via the potential miRNA-mRNA network. Considering its potential of modulating polarization, M2 macrophages-derived exosomes may be a promising therapeutic agent for SCI repair.

Bone Marrow Mesenchymal Stem Cell-Derived Exosomes Accelerate Functional Recovery After Spinal Cord Injury by Promoting the Phagocytosis of Macrophages to Clean Myelin Debris

Macrophage phagocytosis contributes predominantly to processing central nervous system (CNS) debris and further facilitates neurological function restoration after CNS injury. The aims of this study were to evaluate the effect of bone marrow mesenchymal stem cells (BMSC)-derived exosomes (BMSC-Exos) on the phagocytic capability of macrophages to clear myelin debris and to investigate the underlying molecular mechanism during the spinal cord injury (SCI) process. This work reveals that monocyte-derived macrophages (MDMs) infiltrating into the SCI site could efficiently engulf myelin debris and process phagocytic material. However, the phagocytic ability of macrophages to clear tissue debris is compromised after SCI. The administration of BMSC-Exos as an approach for SCI treatment could rescue macrophage normal function by improving the phagocytic capability of myelin debris internalization, which is beneficial for SCI repair, as evidenced by better axon regrowth and increased hindlimb locomotor functional recovery in a rodent model. Examination of macrophage treatment with BMSC-Exos revealed that BMSC-Exos could promote the capacity of macrophages to phagocytose myelin debris in vitro and could create a regenerative microenvironment for axon regrowth. In addition, we confirmed that BMSC-Exo treatment resulted in improved phagocytosis of engulfed myelin debris by promoting the expression of macrophage receptor with collagenous structure (MARCO) in macrophages. The inhibition of MARCO with PolyG (a MARCO antagonist) impaired the effect of BMSC-Exos on the phagocytic capacity of macrophages and resulted in compromised myelin clearance at the lesion site, leading to further tissue damage and impaired functional healing after SCI. In conclusion, these data indicated that targeting the phagocytic ability of macrophages may have therapeutic potential for the improvement in functional healing after SCI. The administration of BMSC-Exos as a cell-free immune therapy strategy has wide application prospects for SCI treatment.

Therapeutic Effects of Azithromycin on Spinal Cord Injury in Male Wistar Rats: A Role for Inflammatory Pathways

BACKGROUND

 Inflammatory responses, including macrophages/microglia imbalance, are associated with spinal cord injury (SCI) complications. Accumulating evidence also suggests an anti-inflammatory property of azithromycin (AZM).

MATERIAL AND METHODS

 Male Wistar rats were subjected to T9 vertebra laminectomy. SCI was induced by spinal cord compression at this level with an aneurysmal clip for 60 seconds. They were divided into three groups: the sham-operated group and two SCI treatment (normal saline as a vehicle control vs. AZM at 180 mg/kg/d intraperitoneally for 3 days postsurgery; first dose: 30 minutes after surgery) groups. Locomotor scaling and behavioral tests for neuropathic pain were evaluated and compared through a 28-day period. At the end of the study, tissue samples were taken to assess neuroinflammatory changes and neural demyelination using ELISA and histopathologic examinations, respectively. In addition, the proportion of M1/M2 macrophage polarization was assessed by using flow cytometry.

RESULTS

 Post-SCI AZM treatment (180 mg/kg/d for 3 days) significantly improved locomotion (p < 0.01) and decreased sensitivity to mechanical (p < 0.01) and thermal allodynia (p < 0.001). Moreover, there was a significant tumor necrosis factor-α (TNF-α) decline (p < 0.01) and interleukin-10 (IL-10) elevation (p < 0.01) in the spinal cord tissue of the AZM-treated group compared with the control groups 28 days post-SCI. AZM significantly improved neuroinflammation as evidenced by reduction of the M1 expression, elevation of M2 macrophages, and reduction of the M1/M2 ratio in both the dorsal root ganglion and the spinal cord tissue after SCI compared with controls (p < 0.01).

CONCLUSION

 AZM treatment can be considered a therapeutic agent for SCI, as it could reduce neuroinflammation and SCI sensory/locomotor complications.

Deciphering glial scar after spinal cord injury

Spinal cord injury (SCI) often leads to permanent disability, which is mainly caused by the loss of functional recovery. In this review, we aimed to investigate why the healing process is interrupted. One of the reasons for this interruption is the formation of a glial scar around the severely damaged tissue, which is usually covered by reactive glia, macrophages and fibroblasts. Aiming to clarify this issue, we summarize the latest research findings pertaining to scar formation, tissue repair, and the divergent roles of blood-derived monocytes/macrophages, ependymal cells, fibroblasts, microglia, oligodendrocyte progenitor cells (OPCs), neuron-glial antigen 2 (NG2) and astrocytes during the process of scar formation, and further analyse the contribution of these cells to scar formation. In addition, we recapitulate the development of therapeutic treatments targeting glial scar components. Altogether, we aim to present a comprehensive decoding of the glial scar and explore potential therapeutic strategies for improving functional recovery after SCI.

Retinoic acid treatment recruits macrophages and increases axonal regeneration after optic nerve injury in the frog Rana pipiens

Retinoic acid (RA) plays major roles during nervous system development, and during regeneration of the adult nervous system. We have previously shown that components of the RA signaling pathway are upregulated after optic nerve injury, and that exogenous application of all-trans retinoic acid (ATRA) greatly increases the survival of axotomized retinal ganglion cells (RGCs). The objective of the present study is to investigate the effects of ATRA application on the macrophages in the optic nerve after injury, and to determine whether this affects axonal regeneration. The optic nerve was crushed and treated with PBS, ATRA and/or clodronate-loaded liposomes. Nerves were examined at one and two weeks after axotomy with light microscopy, immunocytochemistry and electron microscopy. ATRA application to the optic nerve caused transient increases in the number of macrophages and microglia one week after injury. The macrophages are consistently labeled with M2-type markers, and have considerable phagocytic activity. ATRA increased ultrastructural features of ongoing phagocytic activity in macrophages at one and two weeks. ATRA treatment also significantly increased the numbers of regenerating GAP-43-labeled axons. Clodronate liposome treatment depleted macrophage numbers by 80%, completely eliminated the ATRA-mediated increase in axonal regeneration, and clodronate treatment alone decreased axonal numbers by 30%. These results suggest that the success of axon regeneration is partially dependent on the presence of debris-phagocytosing macrophages, and that the increases in regeneration caused by ATRA are in part due to their increased numbers. Further studies will examine whether macrophage depletion affects RGC survival.

Taurolithocholic acid but not tauroursodeoxycholic acid rescues phagocytosis activity of bone marrow-derived macrophages under inflammatory stress

Spinal cord injury (SCI) causes cell death and consequently the breakdown of axons and myelin. The accumulation of myelin debris at the lesion site induces inflammation and blocks axonal regeneration. Hematogenous macrophages contribute to the removal of myelin debris. In this study, we asked how the inflammatory state of macrophages affects their ability to phagocytose myelin. Bone marrow‐derived macrophages (BMDM) and Raw264.7 cells were stimulated with lipopolysaccharides (LPS) or interferon gamma (IFNγ), which induce inflammatory stress, and the endocytosis of myelin was examined. We found that activation of the TLR4‐NFκB pathway reduced myelin uptake by BMDM, while IFNγ‐Jak/STAT1 signaling did not. Since bile acids regulate lipid metabolism and in some cases reduce inflammation, our second objective was to investigate whether myelin clearance could be improved with taurolithocholic acid (TLCA), tauroursodeoxycholic acid or hyodeoxycholic acid. In BMDM only TLCA rescued myelin phagocytosis, when this activity was suppressed by LPS. Inhibition of protein kinase A blocked the effect of TLCA, while an agonist of the farnesoid X receptor did not rescue phagocytosis, implicating TGR5‐PKA signaling in the effect of TLCA. To shed light on the mechanism, we measured whether TLCA affected the expression of CD36, triggering receptor on myeloid cells‐2 (TREM2), and Gas6, which are known to be involved in phagocytosis and affected by inflammatory stimuli. Concomitant with an increase in expression of tumour necrosis factor alpha, LPS reduced expression of TREM2 and Gas6 in BMDM, and TLCA significantly diminished this downregulation. These findings suggest that activation of bile acid receptors may be used to improve myelin clearance in neuropathologies.

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.

Fascin-1 is Highly Expressed Specifically in Microglia After Spinal Cord Injury and Regulates Microglial Migration

Recent research indicates that after spinal cord injury (SCI), microglia accumulate at the borders of lesions between astrocytic and fibrotic scars and perform inflammation-limiting and neuroprotective functions, however, the mechanism of microglial migration remains unclear. Fascin-1 is a key actin-bundling protein that regulates cell migration, invasion and adhesion, but its role during SCI has not been reported. Here, we found that at 7–14 days after SCI in mice, Fascin-1 is significantly upregulated, mainly distributed around the lesion, and specifically expressed in CX3CR1-positive microglia. However, Fascin-1 is not expressed in GFAP-positive astrocytes, NeuN-positive neurons, NG2-positive cells, PDGFRβ-positive cells, or blood-derived Mac2-positive macrophages infiltrating into the lesion core. The expression of Fascin-1 is correspondingly decreased after microglia are specifically depleted in the injured spinal cord by the colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622. The upregulation of Fascin-1 expression is observed when microglia are activated by myelin debris in vitro, and microglial migration is prominently increased. The inhibition of Fascin-1 expression using small interfering RNA (siRNA) markedly suppresses the migration of microglia, but this effect can be reversed by treatment with myelin. The M1/M2-like polarization of microglia does not affect the expression of Fascin-1. Together, our results suggest that Fascin-1 is highly expressed specifically in microglia after SCI and can play an important role in the migration of microglia and the formation of microglial scars. Hence, the elucidation of this mechanism will provide novel therapeutic targets for the treatment of SCI.

Traumatic Brain Injury: An Age-Dependent View of Post-Traumatic Neuroinflammation and Its Treatment

Traumatic brain injury (TBI) is a leading cause of death and disability all over the world. TBI leads to (1) an inflammatory response, (2) white matter injuries and (3) neurodegenerative pathologies in the long term. In humans, TBI occurs most often in children and adolescents or in the elderly, and it is well known that immune responses and the neuroregenerative capacities of the brain, among other factors, vary over a lifetime. Thus, age-at-injury can influence the consequences of TBI. Furthermore, age-at-injury also influences the pharmacological effects of drugs. However, the post-TBI inflammatory, neuronal and functional consequences have been mostly studied in experimental young adult animal models. The specificity and the mechanisms underlying the consequences of TBI and pharmacological responses are poorly understood in extreme ages. In this review, we detail the variations of these age-dependent inflammatory responses and consequences after TBI, from an experimental point of view. We investigate the evolution of microglial, astrocyte and other immune cells responses, and the consequences in terms of neuronal death and functional deficits in neonates, juvenile, adolescent and aged male animals, following a single TBI. We also describe the pharmacological responses to anti-inflammatory or neuroprotective agents, highlighting the need for an age-specific approach to the development of therapies of TBI.

M1-type microglia can induce astrocytes to deposit chondroitin sulfate proteoglycan after spinal cord injury

After spinal cord injury (SCI), astrocytes gradually migrate to and surround the lesion, depositing chondroitin sulfate proteoglycan-rich extracellular matrix and forming astrocytic scar, which limits the spread of inflammation but hinders axon regeneration. Meanwhile, microglia gradually accumulate at the lesion border to form microglial scar and can polarize to generate a pro-inflammatory M1 phenotype or an anti-inflammatory M2 phenotype. However, the effect of microglia polarization on astrocytes is unclear. Here, we found that both microglia (CX3CR1⁺) and astrocytes (GFAP⁺) gathered at the lesion border at 14 days post-injury (dpi). The microglia accumulated along the inner border of and in direct contact with the astrocytes. M1-type microglia (iNOS⁺CX3CR1⁺) were primarily observed at 3 and 7 dpi, while M2-type microglia (Arg1⁺CX3CR1⁺) were present at larger numbers at 7 and 14 dpi. Transforming growth factor-β1 (TGFβ1) was highly expressed in M1 microglia in vitro, consistent with strong expression of TGFβ1 by microglia in vivo at 3 and 7 dpi, when they primarily exhibited an M1 phenotype. Furthermore, conditioned media from M1-type microglia induced astrocytes to secrete chondroitin sulfate proteoglycan in vitro. This effect was eliminated by knocking down sex-determining region Y-box 9 (SOX9) in astrocytes and could not be reversed by treatment with TGFβ1. Taken together, our results suggest that microglia undergo M1 polarization and express high levels of TGFβ1 at 3 and 7 dpi, and that M1-type microglia induce astrocytes to deposit chondroitin sulfate proteoglycan via the TGFβ1/SOX9 pathway. The study was approved by the Institutional Animal Care and Use Committee of Anhui Medical University, China (approval No. LLSC20160052) on March 1, 2016.

Research advances of biomaterials-based microenvironment-regulation therapies for repair and regeneration of spinal cord injury

Traumatic spinal cord injury (SCI) usually results in restricted behaviour recovery and even life-changing paralysis, accompanied with numerous complications. Pathologically, the initial injuries trigger a series of secondary injuries, leading to an expansion of lesion site, a mass of neuron loss, and eventual failure of endogenous axon regeneration. As the advances rapidly spring up in regenerative medicine and tissue engineering biomaterials, regulation of these secondary injuries becomes possible, shedding a light on normal functional restoration. The successful tissue regeneration lies in proper regulation of the inflammatory microenvironment, including the inflammatory immune cells and inflammatory factors that lead to oxidative stress, inhibitory glial scar and neuroexcitatory toxicity. Specifically, the approaches based on microenvironment-regulating biomaterials have shown great promise in the repair and regeneration of SCI. In this review, the pathological inflammatory microenvironments of SCI are discussed, followed by the introduction of microenvironment-regulating biomaterials in terms of their impressive therapeutic effect in attenuation of secondary inflammation and promotion of axon regrowth. With the emphasis on regulating secondary events, the biomaterials for SCI treatment will become promising for clinical applications.

Fibrotic Scar After Spinal Cord Injury: Crosstalk With Other Cells, Cellular Origin, Function, and Mechanism

The failure of axonal regeneration after spinal cord injury (SCI) results in permanent loss of sensorimotor function. The persistent presence of scar tissue, mainly fibrotic scar and astrocytic scar, is a critical cause of axonal regeneration failure and is widely accepted as a treatment target for SCI. Astrocytic scar has been widely investigated, while fibrotic scar has received less attention. Here, we review recent advances in fibrotic scar formation and its crosstalk with other main cellular components in the injured core after SCI, as well as its cellular origin, function, and mechanism. This study is expected to provide an important basis and novel insights into fibrotic scar as a treatment target for SCI.

The Neuroprotective Role of Reactive Astrocytes after Central Nervous System Injury

Reactive astrocytes have traditionally been viewed as a significant contributor to secondary neuronal damage and repair inhibition after central nervous system (CNS) injury attributed, in large part, to their roles in glial scarring. However, more recent transcriptional evidence has uncovered the vast diversity in reactive astrocyte identity and functions that comprises both neuroprotective and -toxic characteristics. Additionally, the capacity of reactive astrocytes to shift between these activation states demonstrates a high level of environment-dependent plasticity that drives the interplay between neuroprotection and -toxicity after CNS injury. These recent findings have spawned a new field of research that seeks to identify and categorize the function of these discrete subpopulations in the context of neurotrauma, as well as identify their regulators. Therefore, this review will discuss the major and most recent advances in this field of research, with a primary emphasis on neuroprotection. This review will also discuss the major pitfalls present in the field, with a particular focus on model species and their impact on the development of novel therapies.

High mobility group box-1 serves a pathogenic role in spinal cord injury via the promotion of pro-inflammatory cytokines

Traumatic spinal cord injury (SCI) is a devastating condition marked by permanent motor, sensory, and autonomic dysfunction, in which the inflammatory response serves an important and preventable role. High mobility group box-1 (HMGB1) is a potent regulator of inflammation in numerous acute and chronic inflammatory conditions.; however, the role of HMGB1 in SCI remains unclear. The present study aimed to characterize the temporal dynamics of HMGB1 release after SCI, to investigate the role of spinal microglia activation in mediating the effects of HMGB1 on SCI, and to explore the therapeutic potential of intrathecal anti-HMGB1 polyclonal antibody on alleviating SCI. The present study demonstrated that HMGB1 expression was increased immediately after traumatic injury of a primary spinal neuron culture. It was found that neutralizing HMGB1 significantly ameliorated SCI pathogenesis and hind limb paralysis. Moreover, the levels of a number of pro-inflammatory cytokines in the SCI lesion were reduced when local HMGB1 was blocked by anti-HMGB1 antibody. In addition, the injured neuron-derived conditioned medium increased TNF-α secretion and the NF-κB pathway in the BV2 microglia cell line via HMGB1. Collectively, these results indicated that HMGB1 served an important role in SCI inflammation and suggested the therapeutic potential of an anti-HMGB1 antibody for SCI.