Macrophage biology in the peripheral nervous system after injury

Perspective on Schwann Cells Derived from Induced Pluripotent Stem Cells in Peripheral Nerve Tissue Engineering

Schwann cells play a crucial role in successful peripheral nerve repair and regeneration by supporting both axonal growth and myelination. Schwann cells are therefore a feasible option for cell therapy treatment of peripheral nerve injury. However, sourcing human Schwann cells at quantities required for development beyond research is challenging. Due to their availability, rapid in vitro expansion, survival, and integration within the host tissue, stem cells have attracted considerable attention as candidate cell therapies. Among them, induced pluripotent stem cells (iPSCs) with the associated prospects for personalized treatment are a promising therapy to take the leap from bench to bedside. In this critical review, we firstly focus on the current knowledge of the Schwann cell phenotype in regard to peripheral nerve injury, including crosstalk with the immune system during peripheral nerve regeneration. Then, we review iPSC to Schwann cell derivation protocols and the results from recent in vitro and in vivo studies. We finally conclude with some prospects for the use of iPSCs in clinical settings.

Effects of Pacap on Schwann Cells: Focus on Nerve Injury

Schwann cells, the most abundant glial cells of the peripheral nervous system, represent the key players able to supply extracellular microenvironment for axonal regrowth and restoration of myelin sheaths on regenerating axons. Following nerve injury, Schwann cells respond adaptively to damage by acquiring a new phenotype. In particular, some of them localize in the distal stump to form the Bungner band, a regeneration track in the distal site of the injured nerve, whereas others produce cytokines involved in recruitment of macrophages infiltrating into the nerve damaged area for axonal and myelin debris clearance. Several neurotrophic factors, including pituitary adenylyl cyclase-activating peptide (PACAP), promote survival and axonal elongation of injured neurons. The present review summarizes the evidence existing in the literature demonstrating the autocrine and/or paracrine action exerted by PACAP to promote remyelination and ameliorate the peripheral nerve inflammatory response following nerve injury.

Liposomes embedded within fibrin gels facilitate localized macrophage manipulations within nerve

BACKGROUND

Understanding the role of macrophages at discrete spatial locations during nerve regeneration after injury is important. But, methodologies that systemically manipulate macrophages can obscure their roles within discrete spatial locations within nerve.

NEW METHOD

Liposomes were embedded within fibrin gels to construct a delivery system that facilitated macrophage-specific manipulations at a sole spatial region, as macrophages accumulated within the fibrin. Clodronate liposomes were characterized for their toxicity to specific cells composing nerve in vitro, then tested for macrophage-specific depletion in vivo. This delivery system using clodronate liposomes was used to repair a mouse sciatic nerve gap to evaluate its efficacy and effects.

RESULT

Clodronate liposomes showed specific toxicity to macrophages without affecting dorsal root ganglia (DRG)-derived neurons, endothelial cells, or Schwann cells in culture. The delivery system demonstrated sustained release of liposomes for more than 7 days while still retaining liposomes within the fibrin. In vivo, the delivery system demonstrated macrophages were targeted by liposomes, and the use of clodronate liposomes minimized macrophage accumulation within fibrin, while not affecting macrophage accumulation within DRG. Nerve regeneration across the nerve gap repaired using this delivery system was associated with decreased angiogenesis, Schwann cell accumulation, axon growth, and reinnervation of affected muscle.

COMPARISON WITH EXISTING METHODS

This delivery system allowed specific perturbation of macrophages locally in nerve. This method could be applicable across species without the need for genetic manipulations or systemic pharmaceuticals.

CONCLUSION

Liposomes embedded within fibrin gels locally target macrophages at the site of nerve injury, which enables greater precision in conclusions regarding their roles in nerve.

Transient Activation of the Hedgehog-Gli Pathway Rescues Radiotherapy-Induced Dry Mouth via Recovering Salivary Gland Resident Macrophages

Irreversible hypofunction of salivary glands is a common side effect of radiotherapy for head and neck cancer and is difficult to remedy. Recent studies indicate that transient activation of Hedgehog signaling rescues irradiation-impaired salivary function in animal models, but the underlying mechanisms are largely unclear. Here, we show in mice that activation of canonical Gli-dependent Hedgehog signaling by Gli1 gene transfer is sufficient to recover salivary function impaired by irradiation. Salivary gland cells responsive to Hedgehog/Gli signaling comprised small subsets of macrophages, epithelial cells, and endothelial cells, and their progeny remained relatively rare long after irradiation and transient Hedgehog activation. Quantities and activities of salivary gland resident macrophages were substantially and rapidly impaired by irradiation and restored by Hedgehog activation. Conversely, depletion of salivary gland macrophages by clodronate liposomes compromised the restoration of irradiation-impaired salivary function by transient Hedgehog activation. Single-cell RNA sequencing and qRT-PCR of sorted cells indicated that Hedgehog activation greatly enhances paracrine interactions between salivary gland resident macrophages, epithelial progenitors, and endothelial cells through Csf1, Hgf, and C1q signaling pathways. Consistently, expression of these paracrine factors and their receptors in salivary glands decreased following irradiation but were restored by transient Hedgehog activation. These findings reveal that resident macrophages and their prorepair paracrine factors are essential for the rescue of irradiation-impaired salivary function by transient Hedgehog activation and are promising therapeutic targets of radiotherapy-induced irreversible dry mouth. SIGNIFICANCE: These findings illuminate a novel direction for developing effective treatment of irreversible dry mouth, which is common after radiotherapy for head and neck cancer and for which no effective treatments are available. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/24/5531/F1.large.jpg.See related commentary by Coppes, p. 5462.

Dexmedetomidine alleviates non-ventilation associated lung injury via modulating immunology phenotypes of macrophages

AIMS

We aimed to evaluate the effect of Dexmedetomidine (Dex) on immunology function of macrophages and inflammatory reactions in non-ventilated lung tissues from both humans and rats.

MAIN METHODS

Patients scheduled for lung lobectomy were randomly assigned to traditional anesthesia group or Dex anesthesia group, 15 subjects in each group. CD68, CD86 and CD206 were used to mark activate and polarized macrophages using immunofluorescence staining in human lung tissues. Sprague-Dawley rats were used to set lung injury model and randomly divided into Control group, one-lung ventilation group (CLI group) and CLI + Dex group. Lung tissues and bronchoalveolar lavage fluid (BALF) from non-ventilated lungs were collected. The acquired lung tissues were subjected to hematoxylin-eosin (H&E) staining and the inflammatory cells in BALF were calculated. Levels of cytokines and chemokines were detected by enzyme-linked immunosorbent assays (ELISA).

KEY FINDINGS

Results from humans showed that anesthesia with Dex decreased the number of both CD68 positive cells and CD86 positive cells and down-regulated level of pro-inflammatory cytokines tumor necrosis factor-α (TNF-α) and monocyte chemotactic protein 1 (MCP-1) in human lung. Results from rats demonstrated that treatment with Dex reversed the increased inflammatory cells in lung and the increased levels of TNF-α, interleukin-1β (IL-β), MCP-1 and chemokine (C-X-C motif) ligand 1 (CXCL1) resulted from non-ventilation; Dex increased the anti-inflammatory cytokine interleukin-10 (IL-10) in BALF from non-ventilated lung.

SIGNIFICANCE

This study showed that Dex modulated the activation and immunological function of macrophages in non-ventilated lung and revealed a protective role in collapsed lung injury.

Localized sympathectomy reduces peripheral nerve regeneration and pain behaviors in 2 rat neuropathic pain models

Previous studies have shown that the peripheral nerve regeneration process is linked to pain in several neuropathic pain models. Other studies show that sympathetic blockade may relieve pain in some pain models and clinical conditions. This study examined reduction in peripheral nerve regeneration as one possible mechanism for relief of neuropathic pain by sympathetic blockade. A "microsympathectomy," consisting of cutting the gray rami containing sympathetic postganglionic axons where they enter the L4 and L5 spinal nerves, reduced mechanical hypersensitivity in 2 different rat neuropathic pain models. In the spinal nerve ligation model, in which some functional regeneration and reinnervation of the ligated spinal nerve can be observed, microsympathectomy reduced functional and anatomical measures of regeneration as well as expression of growth-associated protein 43 (GAP43), a regeneration-related protein. In the spared nerve injury model, in which functional reinnervation is not possible and the futile regeneration process results in formation of a neuroma, microsympathectomy reduced neuroma formation and GAP43 expression. In both models, microsympathectomy reduced macrophage density in the sensory ganglia and peripheral nerve. This corroborates previous work showing that sympathetic nerves may locally affect immune function. The results further highlight the challenge of improving pain in neuropathic conditions without inhibiting peripheral nerve regeneration that might otherwise be possible and desired.

Origin and Differentiation of Nerve-Associated Macrophages

The mature peripheral nervous system is a steady network structure yet shows remarkable regenerative properties. The interaction of peripheral nerves with myeloid cells has largely been investigated in the context of damage, following trauma or infection. Recently, specific macrophages dedicated to homeostatic peripheral nerves have come into focus. These macrophages are defined by tissue and nerve type, are seeded in part prenatally, and self-maintain via proliferation. Thus, they are markedly distinct from monocyte-derived macrophages invading after local disturbance of nerve integrity. The phenotypic and transcriptional adaptation of macrophages to the discrete nervous niche may exert axon guidance and nerve regeneration and thus contribute to the stability of the peripheral nervous network. Deciphering these conserved macrophage-nerve interactions offers new translational perspectives for chronic diseases of the peripheral nervous system, such as diabetic neuropathy and pain.

Migrating Schwann cells direct axon regeneration within the peripheral nerve bridge

Schwann cells within the peripheral nervous system possess a remarkable regenerative potential. Current research shows that peripheral nerve-associated Schwann cells possess the capacity to promote repair of multiple tissues including peripheral nerve gap bridging, skin wound healing, digit tip repair as well as tooth regeneration. One of the key features of the specialized repair Schwann cells is that they become highly motile. They not only migrate into the area of damaged tissue and become a key component of regenerating tissue but also secrete signaling molecules to attract macrophages, support neuronal survival, promote axonal regrowth, activate local mesenchymal stem cells, and interact with other cell types. Currently, the importance of migratory Schwann cells in tissue regeneration is most evident in the case of a peripheral nerve transection injury. Following nerve transection, Schwann cells from both proximal and distal nerve stumps migrate into the nerve bridge and form Schwann cell cords to guide axon regeneration. The formation of Schwann cell cords in the nerve bridge is key to successful peripheral nerve repair following transection injury. In this review, we first examine nerve bridge formation and the behavior of Schwann cell migration in the nerve bridge, and then discuss how migrating Schwann cells direct regenerating axons into the distal nerve. We also review the current understanding of signals that could activate Schwann cell migration and signals that Schwann cells utilize to direct axon regeneration. Understanding the molecular mechanism of Schwann cell migration could potentially offer new therapeutic strategies for peripheral nerve repair.

Axo-glial interaction in the injured PNS

Axons share a close relationship with Schwann cells, their glial partners in peripheral nerves. An intricate axo-glia network of signals and bioactive molecules regulates the major aspects of nerve development and normal functioning of the peripheral nervous system. Disruptions to these complex axo-glial interactions can have serious neurological consequences, as typically seen in injured nerves. Recent studies in inherited neuropathies have demonstrated that damage to one of the partners in this symbiotic unit ultimately leads to impairment of the other partner, emphasizing the bidirectional influence of axon to glia and glia to axon signaling in these diseases. After physical trauma to nerves, dramatic alterations in the architecture and signaling environment of peripheral nerves take place. Here, axons and Schwann cells respond adaptively to these perturbations and change the nature of their reciprocal interactions, thereby driving the remodeling and regeneration of peripheral nerves. In this review, we focus on the nature and importance of axon-glia interactions in injured nerves, both for the reshaping and repair of nerves after trauma, and in driving pathology in inherited peripheral neuropathies.

Tumor-induced neurogenesis and immune evasion as targets of innovative anti-cancer therapies

Normal cells are hijacked by cancer cells forming together heterogeneous tumor masses immersed in aberrant communication circuits that facilitate tumor growth and dissemination. Besides the well characterized angiogenic effect of some tumor-derived factors; others, such as BDNF, recruit peripheral nerves and leukocytes. The neurogenic switch, activated by tumor-derived neurotrophins and extracellular vesicles, attracts adjacent peripheral fibers (autonomic/sensorial) and neural progenitor cells. Strikingly, tumor-associated nerve fibers can guide cancer cell dissemination. Moreover, IL-1β, CCL2, PGE2, among other chemotactic factors, attract natural immunosuppressive cells, including T regulatory (Tregs), myeloid-derived suppressor cells (MDSCs), and M2 macrophages, to the tumor microenvironment. These leukocytes further exacerbate the aberrant communication circuit releasing factors with neurogenic effect. Furthermore, cancer cells directly evade immune surveillance and the antitumoral actions of natural killer cells by activating immunosuppressive mechanisms elicited by heterophilic complexes, joining cancer and immune cells, formed by PD-L1/PD1 and CD80/CTLA-4 plasma membrane proteins. Altogether, nervous and immune cells, together with fibroblasts, endothelial, and bone-marrow-derived cells, promote tumor growth and enhance the metastatic properties of cancer cells. Inspired by the demonstrated, but restricted, power of anti-angiogenic and immune cell-based therapies, preclinical studies are focusing on strategies aimed to inhibit tumor-induced neurogenesis. Here we discuss the potential of anti-neurogenesis and, considering the interplay between nervous and immune systems, we also focus on anti-immunosuppression-based therapies. Small molecules, antibodies and immune cells are being considered as therapeutic agents, aimed to prevent cancer cell communication with neurons and leukocytes, targeting chemotactic and neurotransmitter signaling pathways linked to perineural invasion and metastasis.

High-salt diet decreases mechanical thresholds in mice that is mediated by a CCR2-dependent mechanism

Background

Though it is well-known that a high-salt diet (HSD) is associated with many chronic diseases, the effects of long-term high-salt intake on physiological functions and homeostasis remain elusive. In this study, we investigated whether and how an HSD affects mouse nociceptive thresholds, and myeloid cell trafficking and activation.

Methods

Healthy C57BL/6 male and female mice were fed an HSD (containing 4% NaCl in chow and 1% NaCl in water) from the time of weaning for 3 to 4 months. Circulating monocytes, nerve macrophages, spinal microglia, and associated inflammatory responses were scrutinized using flow cytometry, immunohistochemistry, and quantitative real-time polymerase chain reaction (qPCR) approaches. Mouse pain sensitivity to mechanical stimuli was monitored with von Frey tests along the experimental duration.

Results

Mice on an HSD have reduced mechanical thresholds. They feel more pain than those on a normal diet (ND), e.g., regular laboratory chow (0.3% NaCl in chow). An HSD induced not only a remarkable expansion of circulating monocytes, CCR2⁺Ly6C^hi inflammatory monocytes in particular, but also an accumulation of CD11b⁺F4/80⁺ macrophages in the peripheral nerves and an activation of Iba-1⁺ spinal microglia. Replacing an HSD with a ND was unable to reverse the HSD-induced mechanical hypersensitivity or rescue the altered immune responses. However, treating HSD-fed mice with a chemokine receptor CCR2 antagonist effectively normalized the pain thresholds and immune cell profile in the periphery and spinal cord. An HSD failed to alter pain thresholds and myeloid cell activation in CCR2-deficient mice. Spinal microglial activation is required for HSD-induced mechanical hypersensitivity in male, but not in female mice.

Conclusion

Overall, this study provides evidence that an HSD has a long-term impact on physiological function. CCR2-mediated cellular response, including myeloid cell trafficking and associated inflammation, plays pivotal roles in salt-dietary modulation of pain sensitivity.

Magnetic resonance imaging of enhanced nerve repair with mesenchymal stem cells combined with microenvironment immunomodulation in neurotmesis

INTRODUCTION

The immuno-microenvironment of injured nerves adversely affects mesenchymal stem cell (MSC) therapy for neurotmesis. Magnetic resonance imaging (MRI) can be used noninvasively to monitor nerve degeneration and regeneration. The aim of this study was to investigate nerve repair after MSC transplantation combined with microenvironment immunomodulation in neurotmesis by using multiparametric MRI.

METHODS

Rats with sciatic nerve transection and surgical coaptation were treated with MSCs combined with immunomodulation or MSCs alone. Serial multiparametric MRI examinations were performed over an 8-week period after surgery.

RESULTS

Nerves treated with MSCs combined with immunomodulation showed better functional recovery, rapid recovery of nerve T2, fractional anisotropy and radial diffusivity values, and more rapid restoration of the fiber tracks than nerves treated with MSCs alone.

DISCUSSION

Transplantation of MSCs in combination with immunomodulation can exert a synergistic repair effect on neurotmesis, which can be monitored by multiparametric MRI.

Synaptic Plasticity on Motoneurons After Axotomy: A Necessary Change in Paradigm

Motoneurons axotomized by peripheral nerve injuries experience profound changes in their synaptic inputs that are associated with a neuroinflammatory response that includes local microglia and astrocytes. This reaction is conserved across different types of motoneurons, injuries, and species, but also displays many unique features in each particular case. These reactions have been amply studied, but there is still a lack of knowledge on their functional significance and mechanisms. In this review article, we compiled data from many different fields to generate a comprehensive conceptual framework to best interpret past data and spawn new hypotheses and research. We propose that synaptic plasticity around axotomized motoneurons should be divided into two distinct processes. First, a rapid cell-autonomous, microglia-independent shedding of synapses from motoneuron cell bodies and proximal dendrites that is reversible after muscle reinnervation. Second, a slower mechanism that is microglia-dependent and permanently alters spinal cord circuitry by fully eliminating from the ventral horn the axon collaterals of peripherally injured and regenerating sensory Ia afferent proprioceptors. This removes this input from cell bodies and throughout the dendritic tree of axotomized motoneurons as well as from many other spinal neurons, thus reconfiguring ventral horn motor circuitries to function after regeneration without direct sensory feedback from muscle. This process is modulated by injury severity, suggesting a correlation with poor regeneration specificity due to sensory and motor axons targeting errors in the periphery that likely render Ia afferent connectivity in the ventral horn nonadaptive. In contrast, reversible synaptic changes on the cell bodies occur only while motoneurons are regenerating. This cell-autonomous process displays unique features according to motoneuron type and modulation by local microglia and astrocytes and generally results in a transient reduction of fast synaptic activity that is probably replaced by embryonic-like slow GABA depolarizations, proposed to relate to regenerative mechanisms.

Sigma-1 receptors control neuropathic pain and macrophage infiltration into the dorsal root ganglion after peripheral nerve injury

Neuron-immune interaction in the dorsal root ganglia (DRG) plays a pivotal role in the neuropathic pain development after nerve injury. Sigma-1 receptor (Sig-1R) is expressed by DRG neurons but its role in neuropathic pain is not fully understood. We investigated the effect of peripheral Sig-1R on neuroinflammation in the DRG after spared (sciatic) nerve injury (SNI) in mice. Nerve injury induced a decrease in NeuN staining along with the nuclear eccentricity and ATF3 expression in the injured DRG. Sig-1R was present in all DRG neurons examined, and after SNI this receptor translocated to the periphery of the soma and the vicinity of the nucleus, especially in injured ATF3 + neurons. In WT mice, injured DRG produced the chemokine CCL2, and this was followed by massive infiltration of macrophages/monocytes, which clustered mainly around sensory neurons with translocated Sig-1R, accompanied by robust IL-6 increase and mechanical allodynia. In contrast, Sig-1R knockout (Sig-1R-KO) mice showed reduced levels of CCL2, decreased macrophage/monocyte infiltration into DRG, and less IL-6 and neuropathic mechanical allodynia after SNI. Our findings point to an important role of peripheral Sig-1R in sensory neuron-macrophage/monocyte communication in the DRG after peripheral nerve injury; thus, these receptors may contribute to the neuropathic pain phenotype.

Improved mouse sciatic nerve regeneration following lymphocyte cell therapy

Traumatic injury to the peripheral nervous system (PNS) is the most common cause of acquired nerve damage and impairs the quality of life of patients. The success of nerve regeneration depends on distal stump degeneration, tissue clearance and remodeling, processes in which the immune system participates. We previously reported improved motor recovery in sciatic nerve crush mice following adoptive transfer of lymphocytes, which migrated to the lesion site. However, lymphocyte activity and the nerve tissue response remain unexplored. Thus, in the present study, we evaluated sciatic nerve regeneration and T cell polarization in lymphocyte recipient mice. Splenic lymphocytes were isolated from mice 14 days after sciatic nerve crush and transferred to axotomized animals three days postinjury. Immediate lymphocyte migration to the crushed nerve was confirmed by in vivo imaging. Phenotyping of T helper (Th) cells by flow cytometry revealed an increased frequency of the proinflammatory Th1 and Th17 cell subsets in recipient mice at 7 days and showed that the frequency of these cells remained unchanged for up to 21 days. Moreover, nerve regeneration was improved upon cell therapy, as shown by sustained immunolabeling of axons, Schwann cells, growth-associated protein 43 and BDNF from 14 to 28 days after lesion. Macrophage and IgG immunolabeling were also higher in cell-transferred mice at 14 and 21 days following nerve crush. Functionally, we observed better sensory recovery in the lymphocyte-treated group. Overall, our data demonstrate that enhanced inflammation early after nerve injury has beneficial effects for the regenerative process, improving tissue clearance and axonal regrowth towards the target organs.

Conditioning Electrical Stimulation Is Superior to Postoperative Electrical Stimulation in Enhanced Regeneration and Functional Recovery Following Nerve Graft Repair

Background. Autologous nerve graft is the most common clinical intervention for repairing a nerve gap. However, its regenerative capacity is decreased in part because, unlike a primary repair, the regenerating axons must traverse 2 repair sites. Means to promote nerve regeneration across a graft are needed. Postoperative electrical stimulation (PES) improves nerve growth by reducing staggered regeneration at the coaptation site whereas conditioning electrical stimulation (CES) accelerates axon extension. In this study, we directly compared these electrical stimulation paradigms in a model of nerve autograft repair. Methods. To lay the foundation for clinical translation, regeneration and reinnervation outcomes of CES and PES in a 5-mm nerve autograft model were compared. Sprague-Dawley rats were divided into: ( a) CES, ( b) PES, and ( c) no stimulation cohorts. CES was delivered 1 week prior to nerve cut/coaptation, and PES was delivered immediately following coaptation. Length of nerve regeneration (n = 6/cohort), and behavioral testing (n = 16/cohort) were performed at 14 days and 6 to 14 weeks post-coaptation, respectively. Results. CES treated axons extended 5.9 ± 0.2 mm, significantly longer than PES (3.8 ± 0.2 mm), or no stimulation (2.5 ± 0.2 mm) ( P < .01). Compared with PES animals, the CES animals had significantly improved sensory recovery (von Frey filament testing, intraepidermal nerve fiber reinnervation) ( P < .001) and motor reinnervation (horizontal ladder, gait analysis, nerve conduction studies, neuromuscular junction analysis) ( P < .01). Conclusion. CES resulted in faster regeneration through the nerve graft and improved sensorimotor recovery compared to all other cohorts. It is a promising treatment to improve outcomes in patients undergoing nerve autograft repair.

Cytotoxic Immunity in Peripheral Nerve Injury and Pain

Cytotoxicity and consequent cell death pathways are a critical component of the immune response to infection, disease or injury. While numerous examples of inflammation causing neuronal sensitization and pain have been described, there is a growing appreciation of the role of cytotoxic immunity in response to painful nerve injury. In this review we highlight the functions of cytotoxic immune effector cells, focusing in particular on natural killer (NK) cells, and describe the consequent action of these cells in the injured nerve as well as other chronic pain conditions and peripheral neuropathies. We describe how targeted delivery of cytotoxic factors via the immune synapse operates alongside Wallerian degeneration to allow local axon degeneration in the absence of cell death and is well-placed to support the restoration of homeostasis within the nerve. We also summarize the evidence for the expression of endogenous ligands and receptors on injured nerve targets and infiltrating immune cells that facilitate direct neuro-immune interactions, as well as modulation of the surrounding immune milieu. A number of chronic pain and peripheral neuropathies appear comorbid with a loss of function of cellular cytotoxicity suggesting such mechanisms may actually help to resolve neuropathic pain. Thus while the immune response to peripheral nerve injury is a major driver of maladaptive pain, it is simultaneously capable of directing resolution of injury in part through the pathways of cellular cytotoxicity. Our growing knowledge in tuning immune function away from inflammation toward recovery from nerve injury therefore holds promise for interventions aimed at preventing the transition from acute to chronic pain.

Changes in the transcriptional fingerprint of satellite glial cells following peripheral nerve injury

Satellite glial cells (SGCs) are homeostatic cells enveloping the somata of peripheral sensory and autonomic neurons. A wide variety of neuronal stressors trigger activation of SGCs, contributing to, for example, neuropathic pain through modulation of neuronal activity. However, compared to neurons and other glial cells of the nervous system, SGCs have received modest scientific attention and very little is known about SGC biology, possibly due to the experimental challenges associated with studying them in vivo and in vitro. Utilizing a recently developed method to obtain SGC RNA from dorsal root ganglia (DRG), we took a systematic approach to characterize the SGC transcriptional fingerprint by using next-generation sequencing and, for the first time, obtain an overview of the SGC injury response. Our RNA sequencing data are easily accessible in supporting information in Excel format. They reveal that SGCs are enriched in genes related to the immune system and cell-to-cell communication. Analysis of SGC transcriptional changes in a nerve injury-paradigm reveal a differential response at 3 days versus 14 days postinjury, suggesting dynamic modulation of SGC function over time. Significant downregulation of several genes linked to cholesterol synthesis was observed at both time points. In contrast, regulation of gene clusters linked to the immune system (MHC protein complex and leukocyte migration) was mainly observed after 14 days. Finally, we demonstrate that, after nerve injury, macrophages are in closer physical proximity to both small and large DRG neurons, and that previously reported injury-induced proliferation of SGCs may, in fact, be proliferating macrophages.

Advances in the repair of segmental nerve injuries and trends in reconstruction

Despite advances in surgery, the reconstruction of segmental nerve injuries continues to pose challenges. In this review, current neurobiology regarding regeneration across a nerve defect is discussed in detail. Recent findings include the complex roles of nonneuronal cells in nerve defect regeneration, such as the role of the innate immune system in angiogenesis and how Schwann cells migrate within the defect. Clinically, the repair of nerve defects is still best served by using nerve autografts with the exception of small, noncritical sensory nerve defects, which can be repaired using autograft alternatives, such as processed or acellular nerve allografts. Given current clinical limits for when alternatives can be used, advanced solutions to repair nerve defects demonstrated in animals are highlighted. These highlights include alternatives designed with novel topology and materials, delivery of drugs specifically known to accelerate axon growth, and greater attention to the role of the immune system.

Tailoring synthetic polymeric biomaterials towards nerve tissue engineering: a review

The nervous system is known as a crucial part of the body and derangement in this system can cause potentially lethal consequences or serious side effects. Unfortunately, the nervous system is unable to rehabilitate damaged regions following seriously debilitating disorders such as stroke, spinal cord injury and brain trauma which, in turn, lead to the reduction of quality of life for the patient. Major challenges in restoring the damaged nervous system are low regenerative capacity and the complexity of physiology system. Synthetic polymeric biomaterials with outstanding properties such as excellent biocompatibility and non-immunogenicity find a wide range of applications in biomedical fields especially neural implants and nerve tissue engineering scaffolds. Despite these advancements, tailoring polymeric biomaterials for design of a desired scaffold is fundamental issue that needs tremendous attention to promote the therapeutic benefits and minimize adverse effects. This review aims to (i) describe the nervous system and related injuries. Then, (ii) nerve tissue engineering strategies are discussed and (iii) physiochemical properties of synthetic polymeric biomaterials systematically highlighted. Moreover, tailoring synthetic polymeric biomaterials for nerve tissue engineering is reviewed.

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

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

Distinct VIP and PACAP Functions in the Distal Nerve Stump During Peripheral Nerve Regeneration

Vasoactive Intestinal Peptide (VIP) and Pituitary Adenylyl Cyclase Activating Peptide (PACAP) are regeneration-associated neuropeptides, which are up-regulated by neurons following peripheral nerve injury. So far, they have only been studied for their roles as autocrine signals for both neuronal survival and axon outgrowth during peripheral nerve regeneration. In this report, we examined VIP and PACAP's paracrine effects on Schwann cells and macrophages in the distal nerve stump during peripheral nerve regeneration. We show that VPAC1, VPAC2, and PAC1 are all up-regulated in the mouse distal nerve following peripheral nerve injury and are highly expressed in Schwann cells and macrophages within the distal sciatic nerve. We further investigated the effect of VIP and PACAP on cultured rat Schwann cells, and found that VIP and PACAP can not only promote myelin gene expression in Schwann cells but can also inhibit the release of pro-inflammatory cytokines by Schwann cells. Furthermore, we show that VIP and PACAP inhibit the release of pro-inflammatory cytokines and enhance anti-inflammatory cytokine expression in sciatic nerve explants. Our results provide evidence that VIP and PACAP could have important functions in the distal nerve stump following injury to promote remyelination and regulate the inflammatory response. Thus, VIP and PACAP receptors appear as important targets to promote peripheral nerve repair following injury.

Analysis of Schwann Cell Migration and Axon Regeneration Following Nerve Injury in the Sciatic Nerve Bridge

While it is proposed that interaction between Schwann cells and axons is key for successful nerve regeneration, the behavior of Schwann cells migrating into a nerve gap following a transection injury and how migrating Schwann cells interact with regenerating axons within the nerve bridge has not been studied in detail. In this study, we combine the use of our whole-mount sciatic nerve staining with the use of a proteolipid protein-green fluorescent protein (PLP-GFP) mouse model to mark Schwann cells and have examined the behavior of migrating Schwann cells and regenerating axons in the sciatic nerve gap following a nerve transection injury. We show here that Schwann cell migration from both nerve stumps starts later than the regrowth of axons from the proximal nerve stump. The first migrating Schwann cells are only observed 4 days following mouse sciatic nerve transection injury. Schwann cells migrating from the proximal nerve stump overtake regenerating axons on day 5 and form Schwann cell cords within the nerve bridge by 7 days post-transection injury. Regenerating axons begin to attach to migrating Schwann cells on day 6 and then follow their trajectory navigating across the nerve gap. We also observe that Schwann cell cords in the nerve bridge are not wide enough to guide all the regenerating axons across the nerve bridge, resulting in regenerating axons growing along the outside of both proximal and distal nerve stumps. From this analysis, we demonstrate that Schwann cells play a crucial role in controlling the directionality and speed of axon regeneration across the nerve gap. We also demonstrate that the use of the PLP-GFP mouse model labeling Schwann cells together with the whole sciatic nerve axon staining technique is a useful research model to study the process of peripheral nerve regeneration.

Rapid Isolation of Dorsal Root Ganglion Macrophages

There are growing interests to study the molecular and cellular interactions among immune cells and sensory neurons in the dorsal root ganglia after peripheral nerve injury. Peripheral monocytic cells, including macrophages, are known to respond to a tissue injury through phagocytosis, antigen presentation, and cytokine release. Emerging evidence has implicated the contribution of dorsal root ganglia macrophages to neuropathic pain development and axonal repair in the context of nerve injury. Rapidly phenotyping (or "rapid isolation of") the response of dorsal root ganglia macrophages in the context of nerve injury is desired to identify the unknown neuroimmune factors. Here we demonstrate how our lab rapidly and effectively isolates macrophages from the dorsal root ganglia using an enzyme-free mechanical dissociation protocol. The samples are kept on ice throughout to limit cellular stress. This protocol is far less time consuming compared to the standard enzymatic protocol and has been routinely used for our Fluorescence-activated Cell Sorting analysis.

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

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

Magnetic Composite Biomaterials for Neural Regeneration

Nervous system damage caused by physical trauma or degenerative diseases can result in loss of sensory and motor function for patients. Biomaterial interventions have shown promise in animal studies, providing contact guidance for extending neurites or sustained release of various drugs and growth factors; however, these approaches often target only one aspect of the regeneration process. More recent studies investigate hybrid approaches, creating complex materials that can reduce inflammation or provide neuroprotection in addition to stimulating growth and regeneration. Magnetic materials have shown promise in this field, as they can be manipulated non-invasively, are easily functionalized, and can be used to mechanically stimulate cells. By combining different types of biomaterials (hydrogels, nanoparticles, electrospun fibers) and incorporating magnetic elements, magnetic materials can provide multiple physical and chemical cues to promote regeneration. This review, for the first time, will provide an overview of design strategies for promoting regeneration after neural injury with magnetic biomaterials.

Nociceptor Deletion of Tsc2 Enhances Axon Regeneration by Inducing a Conditioning Injury Response in Dorsal Root Ganglia

Neurons of the PNS are able to regenerate injured axons, a process requiring significant cellular resources to establish and maintain long-distance growth. Genetic activation of mTORC1, a potent regulator of cellular metabolism and protein translation, improves axon regeneration of peripheral neurons by an unresolved mechanism. To gain insight into this process, we activated mTORC1 signaling in mouse nociceptors via genetic deletion of its negative regulator Tsc2. Perinatal deletion of Tsc2 in nociceptors enhanced initial axon growth after sciatic nerve crush, however by 3 d post-injury axon elongation rate became similar to controls. mTORC1 inhibition prior to nerve injury was required to suppress the enhanced axon growth. Gene expression analysis in purified nociceptors revealed that Tsc2-deficient nociceptors had increased activity of regeneration-associated transcription factors (RATFs), including cJun and Atf3, in the absence of injury. Additionally, nociceptor deletion of Tsc2 activated satellite glial cells and macrophages in the dorsal root ganglia (DRG) in a similar manner to nerve injury. Surprisingly, these changes improved axon length but not percentage of initiating axons in dissociated cultures. The pro-regenerative environment in naïve DRG was recapitulated by AAV8-mediated deletion of Tsc2 in adult mice, suggesting that this phenotype does not result from a developmental effect. Consistently, AAV8-mediated Tsc2 deletion did not improve behavioral recovery after a sciatic nerve crush injury despite initially enhanced axon growth. Together, these data show that neuronal mTORC1 activation induces an incomplete pro-regenerative environment in the DRG that improves initial but not later axon growth after nerve injury.

Application of CNTF or FGF-2 increases the number of M2-like macrophages after optic nerve injury in adult Rana pipiens

We have previously shown that a single application of the growth factors ciliary neurotrophic factor (CNTF) or fibroblast growth factor 2 (FGF-2) to the crushed optic nerve of the frog, Rana pipiens, increases the numbers and elongation rate of regenerating retinal ganglion cell axons. Here we investigate the effects of these factors on the numbers and types of macrophages that invade the regeneration zone. In control PBS-treated nerves, many macrophages are present 100 μm distal to the crush site at 1 week after injury; their numbers halve by 2 weeks. A single application of CNTF at the time of injury triples the numbers of macrophages at 1 week, with this increase compared to control being maintained at 2 weeks. Application of FGF-2 is equally effective at 1 week, but the macrophage numbers have fallen to control levels at 2 weeks. Immunostaining with a pan-macrophage marker, ED1, and a marker for M2-like macrophages, Arg-1, showed that the proportion of the putative M2 phenotype remained at approximately 80% with all treatments. Electron microscopy of the macrophages at 1 week shows strong phagocytic activity with all treatments, with many vacuoles containing axon fragments and membrane debris. At 2 weeks with PBS or FGF-2 treatment the remaining macrophages are less phagocytically active, containing mainly lipid inclusions. With CNTF treatment, at 2 weeks many of the more numerous macrophages are still phagocytosing axonal debris, although they also contain lipid inclusions. We conclude that the increase in macrophage influx seen after growth factor application is beneficial for the regenerating axons, probably due to more extensive removal of degenerating distal axons, but also perhaps to secretion of growth-promoting substances.

The origin, fate, and contribution of macrophages to spinal cord injury pathology

Virtually all phases of spinal cord injury pathogenesis, including inflammation, cell proliferation and differentiation, as well as tissue remodeling, are mediated in part by infiltrating monocyte-derived macrophages. It is now clear that these infiltrating macrophages have distinct functions from resident microglia and are capable of mediating both harmful and beneficial effects after injury. These divergent effects have been largely attributed to environmental cues, such as specific cytokines, that influence the macrophage polarization state. In this review, we also consider the possibility that different macrophage origins, including the spleen, bone marrow, and local self-renewal, may also affect macrophage fate, and ultimately their function that contribute to the complex pathobiology of spinal cord injury.