Macrophage biology in the peripheral nervous system after injury

Temporal and spatial expression of Phosphodiesterase-4B after sciatic nerve compression in rats and its mechanism of action on sciatic nerve repair

BACKGROUND

Macrophage phenotype transformation is vital in sciatic nerve injury. The study of biomolecule expression and its impact on macrophage phenotype transformation is a current research focus.

MATERIAL AND METHODS

We created a rat model of sciatic nerve compression injury to examine the expression of PDE4B and the distribution of M1 and M2 macrophages over time and their relationship. We confirmed the effect of inhibiting PDE4B expression on macrophage phenotype changes and its role in sciatic nerve injury repair. The experiments consisted of immunofluorescence, western blotting, HE staining, TEM, and behavioral evaluation. Investigate in vivo experiment results with RAW264.7 cells in vitro. PDE4B knockdown lentivirus was transfected into RAW264.7 cells and stimulated with LPS and IFN-γ. We assessed CD86 and CD206 expression using flow cytometry and western blot. The relationship between PDE4B and the TLR4/NF-κB pathway was studied.

RESULTS

PDE4B peaked on day 7 after surgery, alongside the highest M1 macrophages count. PDE4B and M1 macrophages decreased, and M2 macrophages increased. PDE4B inhibition reduced M1 macrophages, increased M2 macrophages, suppressed inflammation, and promoted sciatic nerve repair while alleviating pain. In vitro experiments confirmed that PDE4B regulated macrophage phenotype via the TLR4/NF-κB pathway. Inhibiting PDE4B disrupted this pathway and promoted M2 macrophage transformation.

CONCLUSIONS

In the sciatic nerve injury, PDE4B expression is linked to the M1 macrophage phenotype. Low PDE4B expression facilitates the M1 to M2 macrophage transformation and supports sciatic nerve repair. The TLR4/NF-κB pathway is involved in this process.

Inhibition of Macrophage Activation by Minocycline Attenuates CCI-Induced Neuropathic Pain

Neuropathic pain is characterized by a high prevalence and associated with a variety of disorders of the peripheral and central nervous systems. It remains a major challenge for clinical management due to lack effective treatments. Our previous studies have demonstrated that nerve injury-induced neuroinflammation plays a critical role in regulating the development and maintenance of neuropathic pain. In the present study, we found that chronic constriction injury (CCI) led to a significant increase in the number of macrophages at the site of injured nerves. To elucidate the role of macrophage activation in CCI-induced neuropathic pain, we employed chemical agents, including clodronate liposomes, which is known for their ability to deplete macrophages, and minocycline, an inhibitor of macrophage function. Both intravenous injection of liposome-encapsulated clodronate and intrasciatic delivery of minocycline effectively attenuated CCI-induced mechanical and heat hyperalgesia. Furthermore, transfer of polarized M2 macrophages significantly alleviated CCI-induced neuropathic pain, but not under the condition of M1 macrophage transfer. Mechanistically, our findings indicated that pretreatment with minocycline increased the expression level of CD206 but decreased that of IL-1β, while post-polarization treatment markedly decreased the expression level of both. Additionally, an in vitro migration assay revealed that minocycline exerts an inhibitory effect on macrophage migration. In brief, our study elucidates the effect of CCI-induced macrophage activation on neuropathic pain and provides new insights into the potential clinical application of minocycline for managing neuropathic pain.

Lactoferrin-derived peptide PXL01 impacts nerve regeneration after sciatic nerve reconstruction in healthy and diabetic rats

Introduction

Although advanced surgical techniques are available, satisfactory functional outcomes after peripheral nerve injuries are uncommon. Hence, immune-modulating factors such as PXL01, a lactoferrin-derived peptide that improves axonal outgrowth in injured human digital nerves, have gained attention. We previously reported a short-term immunosuppressive effect of PXL01 after the repair of transected rat sciatic nerves, but it had no effect on nerve regeneration. Here, we investigated the potential of PXL01 to improve nerve regeneration in healthy rats and in a rat model of type 2 diabetes (Goto-Kakizaki [GK] rats).

Methods

A 10-mm sciatic nerve defect was created in healthy (n = 14) and diabetic GK rats (n = 14) and reconstructed using nerve autografts. Immediately after surgery, PXL01 or sodium chloride (control, placebo) (n = 7 for each treatment) was administered around the autograft. On day 8, immunohistochemical staining of the sciatic nerve and dorsal root ganglia (DRGs) was performed to analyze axonal outgrowth (neurofilament staining); inflammation (CD68 and CD206 macrophage staining in nerve); Schwann cell and sensory neuron activation (transcription factor ATF3 staining in nerve and DRGs) and apoptosis (cleaved caspase 3 staining in nerve); and neuroprotection (heat shock protein [HSP27] staining in nerve and DRGs).

Results

PXL01 had no impact on the macrophage response in the autografts but increased axonal outgrowth and HSP27 expression in the DRGs of healthy and diabetic rats, despite a lower number of activated Schwann cells in the autograft. Diabetes affected axonal outgrowth, Schwann cell and macrophage responses, and HSP27 expression. These effects were observed in the sciatic nerve as well as the DRG.

Discussion

Application of PXL01, despite having no impact on macrophages, may improve axonal outgrowth and affects Schwann cell activation in autograft-reconstructed sciatic nerves, as well as conveys neuroprotection (HSP27 expression) in the DRGs of healthy and diabetic GK rats. Diabetes influenced nerve regeneration in such autografts. Therefore, PXL01 is a promising candidate to improve nerve regeneration.

TGF-β1 Improves Nerve Regeneration and Functional Recovery After Sciatic Nerve Injury by Alleviating Inflammation

Background: Peripheral nerves have a certain regenerative ability, but their repair and regeneration after injury is a complex process, usually involving a large number of genes and proteins. In a previous study, we analyzed the gene expression profile in rats after sciatic nerve injury and found significant changes in transforming growth factor-beta 1 (TGF-β1) expression, suggesting that TGF-β1 may be involved in the process of nerve regeneration after injury. Methods: In this study, we first detected the time-course expression and localization of TGF-β1 in dorsal root ganglion (DRG) tissues in a rat sciatic nerve transection model via RT-qPCR. Secondly, we investigated the bioactive roles of TGF-β1 in primary cultured DRG neuron cells through a CCK8 assay, TUNEL assay, and immunofluorescence staining. Thirdly, we explored the neuroprotective roles of TGF-β1 in an in vivo model of sciatic nerve regeneration through morphological observation, behavioral, and electrophysiological tests, and a molecular biological measure. Results: We found that TGF-β1 expression was increased after injury and mainly located in the cytoplasm and nuclei of neuron cells in the DRG. TGF-β1 may regulate the viability, apoptosis, and neurite outgrowth of primary DRG neuron cells. In our in vivo model of sciatic nerve regeneration, TGF-β1 improved nerve regeneration and neuronal function recovery after sciatic nerve injury, alleviated the inflammatory response, and relieved neuropathic pain via the TGF-β1/smad2 pathway. Conclusions: This study provides an experimental and theoretical basis for using TGF-β1 as a neuroprotective agent after peripheral nerve injury in clinical practice in the future.

The Role of Macrophages in Nerve Regeneration: Polarization and Combination with Tissue Engineering

Peripheral nerve regeneration after trauma poses a substantial clinical challenge that has already been investigated for many years. Infiltration of immune cells is a critical step in the response to nerve damage that creates a supportive microenvironment for regeneration. In this work, we focus on a special type of immune cell, macrophage, in addressing the problem of neuronal regeneration. We discuss the complex endogenous mechanisms of peripheral nerve injury and regrowth vis-à-vis macrophages, including their recruitment, polarization, and interplay with Schwann cells post-trauma. Furthermore, we elucidate the underlying mechanisms by which exogenous stimuli govern the above events. Finally, we summarize the necessary roles of macrophages in peripheral nerve lesions and reconstruction. There are many challenges in controlling macrophage functions to achieve complete neuronal regeneration, even though considerable progress has been made in understanding the connection between these cells and peripheral nerve damage.

Polyethylene glycol has immunoprotective effects on sciatic allografts, but behavioral recovery and graft tolerance require neurorrhaphy and axonal fusion

JOURNAL/nrgr/04.03/01300535-202504000-00033/figure1/v/2024-07-06T104127Z/r/image-tiff Behavioral recovery using (viable) peripheral nerve allografts to repair ablation-type (segmental-loss) peripheral nerve injuries is delayed or poor due to slow and inaccurate axonal regeneration. Furthermore, such peripheral nerve allografts undergo immunological rejection by the host immune system. In contrast, peripheral nerve injuries repaired by polyethylene glycol fusion of peripheral nerve allografts exhibit excellent behavioral recovery within weeks, reduced immune responses, and many axons do not undergo Wallerian degeneration. The relative contribution of neurorrhaphy and polyethylene glycol-fusion of axons versus the effects of polyethylene glycol per se was unknown prior to this study. We hypothesized that polyethylene glycol might have some immune-protective effects, but polyethylene glycol-fusion was necessary to prevent Wallerian degeneration and functional/behavioral recovery. We examined how polyethylene glycol solutions per se affect functional and behavioral recovery and peripheral nerve allograft morphological and immunological responses in the absence of polyethylene glycol-induced axonal fusion. Ablation-type sciatic nerve injuries in outbred Sprague-Dawley rats were repaired according to a modified protocol using the same solutions as polyethylene glycol-fused peripheral nerve allografts, but peripheral nerve allografts were loose-sutured (loose-sutured polyethylene glycol) with an intentional gap of 1-2 mm to prevent fusion by polyethylene glycol of peripheral nerve allograft axons with host axons. Similar to negative control peripheral nerve allografts not treated by polyethylene glycol and in contrast to polyethylene glycol-fused peripheral nerve allografts, animals with loose-sutured polyethylene glycol peripheral nerve allografts exhibited Wallerian degeneration for all axons and myelin degeneration by 7 days postoperatively and did not recover sciatic-mediated behavioral functions by 42 days postoperatively. Other morphological signs of rejection, such as collapsed Schwann cell basal lamina tubes, were absent in polyethylene glycol-fused peripheral nerve allografts but commonly observed in negative control and loose-sutured polyethylene glycol peripheral nerve allografts at 21 days postoperatively. Loose-sutured polyethylene glycol peripheral nerve allografts had more pro-inflammatory and less anti-inflammatory macrophages than negative control peripheral nerve allografts. While T cell counts were similarly high in loose-sutured-polyethylene glycol and negative control peripheral nerve allografts, loose-sutured polyethylene glycol peripheral nerve allografts expressed some cytokines/chemokines important for T cell activation at much lower levels at 14 days postoperatively. MHCI expression was elevated in loose-sutured polyethylene glycol peripheral nerve allografts, but MHCII expression was modestly lower compared to negative control at 21 days postoperatively. We conclude that, while polyethylene glycol per se reduces some immune responses of peripheral nerve allografts, successful polyethylene glycol-fusion repair of some axons is necessary to prevent Wallerian degeneration of those axons and immune rejection of peripheral nerve allografts, and produce recovery of sensory/motor functions and voluntary behaviors. Translation of polyethylene glycol-fusion technologies would produce a paradigm shift from the current clinical practice of waiting days to months to repair ablation peripheral nerve injuries.

Mineralocorticoid receptor antagonism improves corneal integrity in a rat model of limbal stem cell deficiency

Limbal stem cell deficiency (LSCD) is a sight-threatening condition caused by the loss and/or dysfunction of limbal stem cells (LSCs), which are essential for corneal epithelial regeneration and homeostasis and are critical for maintaining corneal transparency. We have previously shown that specific inactivation of the endothelial mineralocorticoid receptor (MR) inhibits corneal neovascularization (CN) and that MR antagonists (MRA) improve corneal epithelial wound healing. This study investigated the therapeutic potential of MRA in LSCD and their mechanisms of action. Using a rat model of LSCD, systemic administration of spironolactone (SPL) or a more specific MRA, eplerenone, similarly reduced CN and corneal oedema, demonstrating MR-specific effects. SPL further limited inflammation, enhanced the corneal epithelial barrier, reduced corneal conjunctivalization and promoted nerve regeneration, highlighting its potential to improve corneal integrity. Transcriptomic analysis revealed that SPL upregulated genes associated with LSC maintenance (Tp63, Wnt6), corneal epithelial differentiation (Vdr, Fermt1, Ehf) and nerve regeneration (Sprr1a, Anxa1), while downregulating genes associated with angiogenesis (Kdr, Scube2), inflammation (Ccl2, Cxcl1) and fibrosis (Fbln1, Snai1). Conversely, transgenic rats overexpressing human NR3C2 encoding MR showed corneal epithelial irregularities and dysregulation of genes related to extracellular matrix remodeling and fibrosis (Matn3, Serpine2, Fmod, Bgn, Ddr2), angiogenesis (Nrp2, Scube1) and limbal cell function (Ifitm3). These findings demonstrate that activation of the MR pathway disrupts limbal and corneal homeostasis and that SPL effectively modulates critical mechanisms in LSCD, offering promising therapeutic potential to reduce CN and improve corneal epithelial barrier integrity.

Absence of T-box transcription factor 21 limits neuromuscular junction recovery after nerve injury in T-bet-knockout mice

Introduction

Terminal Schwann cells (tSCs), at the neuromuscular junction (NMJ), play critical roles in the repair of motor axon terminals at muscle, and rebuild neuronal signaling following nerve injury. Knowledge of mediators impacting tSCs post-nerve injury and in disease may guide beneficial therapies to improve motor outcomes. We previously found T-box transcription factor 21 (TBX21/TBET), classically associated with T-helper1 cells and immune cell recruitment, is expressed in tSCs at the mouse NMJ. The purpose of this study was to examine effects of Tbx21 absence during NMJ regeneration following peripheral nerve injury.

Methods

Wildtype (WT) and Tbet-knockout (Tbet-KO) mice underwent sciatic nerve transection and immediate repair. Functional muscle recovery assessment was performed with muscle force testing on mice at 2-, 3-, 4-, and 6-week (wks) and 6 months after nerve injury repair. Morphometric analyses of NMJ reinnervation, tSC number, and tSC processes were evaluated. Full NMJ reinnervation was defined as ≥75% coverage of endplates by axons. A minimum of three mice were evaluated in each group, and 50-100 NMJs were evaluated per mouse.

Results

Tbet-KO mice had significantly diminished muscle function compared to WT mice at every time point beyond 3 weeks. Tbet-KO mice showed just over half of the muscle force generated by WT mice at 4 weeks and 6 weeks post-injury and repair. By 6 months, Tbet-KO mice generated only 84.1% the muscle force of WT mice. Tbet-KO mice showed significantly decreased levels of fully reinnervated NMJs compared to WT mice at each time point tested. Tbet-KO mice also showed a lower number of tSCs with reduced cytoplasmic processes beyond NMJ area and lower number of immune cells during process of NMJ regeneration.

Discussion

Our findings show that the Tbx21 transcription factor promotes NMJ reinnervation to regain muscle function following nerve injury.

Immune Cell Biology in Peripheral Nervous System Injury

BACKGROUND

The peripheral nervous system (PNS) exhibits remarkable regenerative capability after injury. PNS regeneration relies on neurons themselves as well as a variety of other cell types, including Schwann cells, immune cells, and non-neuronal cells.

OBJECTIVES

This paper focuses on summarizing the critical roles of immune cells (SCs) in the injury and repair processes of the PNS.

RESULTS

During peripheral nerve injury, macrophages infiltrate the site under the induction of various cytokines, primarily accumulating at the dorsal root ganglia (DRG) and the nerve distal to the injury site, with only a small number detected at the nerve proximal to the injury site. The phenotype of macrophages during injury remains controversial, but recent single-cell sequencing analyses may provide new insights. In peripheral nervous system injury, macrophages participate in Wallerian degeneration as well as in the reconstruction of nerve bridges and angiogenesis during axonal regeneration. Neutrophils appear early in the injury process and are primarily present at the injury site and the distal segment. After peripheral nervous system injury, immature neutrophils from the peripheral blood play a major role. Although lymphocytes constitute only a small fraction compared to macrophages and neutrophils after peripheral nervous system injury, they still play important roles, including Treg cells, B cells, and NK cells. A large number of immune cells accumulate at the injury site, contributing not only to Wallerian degeneration but also to axonal regeneration.

CONCLUSION

In conclusion, this paper summarizes current knowledge regarding the mechanisms of immune cell infiltration after PNS injury, providing new insights for future research on the role of immune cells in peripheral nerve injury.

The Role of 70-MHz Ultrahigh-Frequency Ultrasound in the Peripheral Nerve Injury

AIM

High-frequency ultrasound with an 18-MHz probe (18 MHz-HFUS) plays a relevant role in the evaluation of peripheral nerve injury (PNI). Ultrahigh-frequency ultrasound with a 70-MHz probe (70 MHz-UHFUS) offers higher spatial resolution and could allow a better detection of PNI. This study aimed to compare the diagnostic performance of HFUS and UHFUS in PNI detection.

MATERIALS AND METHODS

In this retrospective study, were selected, between July 2022 and April 2024, 61 patients underwent HFUS, UHFUS, and nerve conduction study (NCS) for clinical suspicion of traumatic forearm PNI. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and diagnostic accuracy of HFUS and UHFUS in PNI detection were calculated and compared. NCS was used as the reference standard. Nonparametric statistical tests were used. A p value of < 0.05 was considered statistically significant.

RESULTS

Comparing the diagnostic performance in PNI detection, the 70 MHz-UHFUS showed a sensitivity and diagnostic accuracy significantly higher than 18 MHz-HFUS, respectively, 98.0% versus 82.4% (p = 0.0205) and 95.1% versus 82.0% (p = 0.0468). Otherwise, not significantly difference were in specificity, PPV, and NPV.

CONCLUSIONS

UHFUS compared to HFUS demonstrated a higher sensitivity and diagnostic accuracy in PNI detection.

Unveiling the Mechanisms of Pain in Endometriosis: Comprehensive Analysis of Inflammatory Sensitization and Therapeutic Potential

Endometriosis is a complicated, estrogen-dependent gynecological condition with a high morbidity rate. Pain, as the most common clinical symptom of endometriosis, severely affects women's physical and mental health and exacerbates socioeconomic burden. However, the specific mechanisms behind the occurrence of endometriosis-related pain remain unclear. It is currently believed that the occurrence of endometriosis pain is related to various factors, such as immune abnormalities, endocrine disorders, the brain-gut axis, angiogenesis, and mechanical stimulation. These factors induce systemic chronic inflammation, which stimulates the nerves and subsequently alters neural plasticity, leading to nociceptive sensitization and thereby causing chronic pain. In this paper, we compile and review the articles published on the study of nociceptive sensitization and endometriosis pain mechanisms. Starting from the factors influencing the chronic pain associated with endometriosis, we explain the relationship between these factors and chronic inflammation and further elaborate on the potential mechanisms by which chronic inflammation induces nociceptive sensitization. We aim to reveal the possible mechanisms of endometriosis pain, as well as nociceptive sensitization, and offer potential new targets for the treatment of endometriosis pain.

Refinement of Protein Extraction Protocols for Human Peripheral Nerve Tissue

Our aim was to establish an effective method for protein extraction from freshly frozen human peripheral nerves, determine the minimum amount required for consistent protein extraction outcomes, and assess which method produced the highest number of protein identities. Five extraction methods were compared using 8 M urea and Ripa buffer using either the Bullet Blender or Bioruptor. Out of the total 2619 identified proteins, protein extraction using the Ripa buffer combined with either Bioruptor or Bullet Blender resulted in the identification of 1582 (60%) and 1615 (62%) proteins, respectively. In contrast, using 8 M urea and Bioruptor for protein extraction resulted in 1022 proteins (39%), whereas employing Bullet Blender yielded 1446 proteins (55%). Sample amounts, ranging from 0.6 to 10 mg, were prepared with consistent protein extraction outcome obtained for samples ≥1.2 mg. Combining Ripa and 8 M urea with Bullet Blender increased protein identification to 2126 (81%). Proteins were classified by their cell components, molecular functions, and biological processes. Furthermore, a subclassification of proteins involved in the extracellular matrix (ECM) was introduced. We recommend the use of Ripa buffer, in combination with 8 M urea and Bullet Blender for extracting proteins from fresh-frozen human nerves weighing ≥1.2 mg.

Tetrahydropalmatine ameliorates peripheral nerve regeneration by enhancing macrophage anti-inflammatory response

BACKGROUND

Peripheral nerve injury (PNI) is a common clinical problem that can result in partial or complete loss of sensory, motor, and autonomic functions. Tetrahydropalmatine (THP), a Corydalis yanhusuo-derived phytochemical alkaloid, possesses hypnotic, soothing, analgesic, and other effects, but little is known about the effect of THP on moderating peripheral nerve regeneration and its possible underlying mechanism of action.

PURPOSE

In this study, we aim to elucidate the protective function of THP on PNI and further reveal the underlying pharmacological mechanisms.

METHODS

PNI rats were in suit injection of THP solution at doses of 40 mg/kg for consecutive 3, 7, or 28 days, followed by harvesting the sciatic nerve tissues. The protective effect of THP on PNI was evaluated by electrophysiological test, transmission electron microscopy, immunofluorescence (IF), and western blotting (WB). Macrophage polarization, the expression of inflammatory-related genes and cytokines, and its upstream signaling pathways were detected by IF, WB, enzyme-linked immunosorbent assay (ELISA), mRNA-seq, and WB. In vitro, the Raw 264.7 cells were treated with lipopolysaccharide containing with/without THP. The degree of inflammatory activation and its potential pharmacological mechanism were measured by ELISA, qRT-PCR, IF staining, flow cytometry, and WB. Additionally, a pharmacological agonist or inhibitor was added to the cell medium to further identify the role of THP's potential pharmacological mechanism in regulating inflammatory response via IF and ELISA technology.

RESULTS

Using the sciatic nerve crush model, we found that THP significantly enhanced the rate of axonal growth and functional recovery, and altered macrophage subtype transformation from the M1/M0 phenotype into the M2 phenotype, inducing the secretion of large amounts of anti-inflammatory factors. Moreover, THP significantly increased the phosphorylation level of PI3K, AKT, GSK3β, and IκBa, and decreased the expression of TLR4 protein and NF-κB phosphorylation. Similarly, in vitro, THP also facilitated Raw 264.7 cell polarization to the M2 subtype under the condition of LPS stimulation. Meanwhile, the change of PI3K/AKT/GSK3β and TLR4/NF-κB signaling-related proteins in vitro was consistent with the results in vivo. Additionally, the THP-medicated anti-inflammatory effect on Raw 264.7 cells was partly eliminated when pharmacological intervention of these two signaling pathways.

CONCLUSIONS

THP has anti-inflammatory effects on facilitating M2-subtype macrophage polarization, which produces abundant anti-inflammatory cytokines to ameliorate peripheral nerve regeneration. Moreover, the potential mechanism of THP action may be intimately associated with activating the PI3K/AKT/GSK3β axis and inhibiting the TLR4/NF-κB pathway.

Nasal mucosal mesenchymal stem cells promote repair of sciatic nerve injury in rats by modulating the inflammatory microenvironment

Sciatic nerve injury (SNI) represents the most prevalent form of peripheral nerve damage, resulting in the rapid activation of macrophages into the M1 phenotype following injury. This activation induces an inflammatory microenvironment that negatively impacts nerve regeneration. Ectodermal mesenchymal stem cells (EMSCs), isolated from nasal mucosa, possess the capacity for multidirectional differentiation and exhibit immunomodulatory effects. Modulating macrophage polarization to create a favorable environment for nerve repair may represent a potential approach to facilitate nerve recovery. This investigation sought to explore the effects of EMSCs transplantation on macrophage polarization and nerve regeneration in SNI, as well as to identify the underlying mechanisms. An in vivo SNI model was established, and behavioral and histological analyses demonstrated that EMSCs transplantation facilitated nerve function recovery. Furthermore, immunofluorescence and Western blot assays revealed an increase in M2 macrophage presence and the secretion of anti-inflammatory cytokines following EMSCs transplantation, thereby promoting nerve regeneration. In vitro, EMSCs were found to enhance M2 macrophage polarization and the production of anti-inflammatory factors. Additionally, it was confirmed that EMSCs regulate macrophage polarization through the PI3K/AKT/NF-κB signaling pathway, thereby fostering an optimal inflammatory environment for nerve regeneration.

CCL2/CCR2 axis promotes perineural invasion of salivary adenoid cystic carcinoma via ITGβ5-mediated nerve-tumor interaction

Perineural invasion (PNI) is a notorious feature of salivary adenoid cystic carcinoma (SACC) and other neurotropic tumors. The pathogenesis of PNI that involves the molecular communication between the tumor and the suffered nerve is elusive. The in vitro co-culture assays of SACC cells with dorsal root ganglia (DRG) or neural cells showed that nerve-derived CCL2 activated CCR2 expression in SACC cells, promoting the proliferation, adhesion, migration, and invasion of SACC cells via the ERK1/2/ITGβ5 pathway. Meanwhile, SACC-derived exosomes delivered ITGβ5 to promote the neurite outgrowth of neural cells or DRG. Blocking of CCL2/CCR2 axis or ITGβ5 inhibited the PNI of SACC cells in models in vitro by 3D co-culture of DRG with SACC cells and in vivo by xenografting SACC cells onto the murine sciatic nerve. High levels of ITGβ5 in tissues or plasma exosomes were significantly correlated with CCL2 and CCR2 expression in the tissues and associated with PNI and poor prognosis of SACC cases. Our findings revealed a novel reciprocal loop between neural and tumor cells driven by the CCL2/CCR2 axis and exosomal ITGβ5 during PNI of SACC. The present study may provide a prospective diagnostic and anti-PNI treatment strategy for SACC patients via targeting the nerve-tumor interactions.

Inhibition of KIF5b-mediated Nav1.8 transport by ropivacaine contributes to axonal regeneration following sciatic nerve injury in rats

Peripheral nerve injury (PNI), typically caused by traumatic accidents or medical events, is currently one of the most common diseases that leads to limb disability. After PNI, tetrodotoxin-resistant voltage-gated sodium channel Nav1.8 is upregulated at the lesion site. Our earlier study suggested that ropivacaine promotes axon regrowth by regulating Nav1.8-mediated macrophage signaling. Nevertheless, the mechanism of ropivacaine in regulation of Nav1.8 expression remains incompletely understood. Kinesin family 5b (KIF5b) was reported to mediate the Nav1.8 axonal transport from dorsal root ganglia (DRGs) to lesion site. Herein, we investigated whether ropivacaine promotes axon regeneration through inhibition of KIF5b-mediated Nav1.8 transport. Reduced levels of KIF5b and Nav1.8 in DRGs coincide with their increase at the lesion site. Nav1.8 mRNA was significantly increased at the lesion site but not in DRGs. Surprisingly, ropivacaine reversed the alterations of Nav1.8 and KIF5b protein expression without affecting Nav1.8 mRNA level. Due to KIF5b overexpression in DRGs, Nav1.8 protein level was significantly decreased in DRGs and increased at the lesion site. We also found KIF5b overexpression significantly impaired behavioral functions, reduced the recovery index of compound muscle action potential (CMAP) amplitude, inhibited axonal regrowth, slowed M1 macrophage infiltration and shift to M2 phenotype, and delayed myelin debris clearance. Notably, all aforementioned results caused by KIF5b overexpression were alleviated by ropivacaine. Furthermore, we demonstrated that inhibition of Nav1.8 transport by A-803467 produced mitigating effects on the impairment of regenerative capacity induced by KIF5b overexpression similar to ropivacaine. These results suggest that ropivacaine promotes axonal regeneration at least partially by inhibiting KIF5b-mediated Nav1.8 forward transport.

Human dorsal root ganglia are either preserved or completely lost after deafferentation by brachial plexus injury

BACKGROUND

Plexus injury results in lifelong suffering from flaccid paralysis, sensory loss, and intractable pain. For this clinical problem, regenerative medicine concepts set high expectations. However, it is largely unknown how dorsal root ganglia (DRG) are affected by accidental deafferentation.

METHODS

Here, we phenotyped DRG of a clinically and MRI-characterised cohort of 13 patients with plexus injury. Avulsed DRG were collected during reconstructive nerve surgery. For control, we used DRG from forensic autopsy. The cellular composition of the DRG was analysed in histopathological slices with multicolour high-resolution immunohistochemistry, tile microscopy, and deep-learning-based bioimage analysis. We then sequenced the bulk RNA of corresponding DRG slices.

RESULTS

In about half of the patients we found loss of the typical DRG units consisting of neurones and satellite glial cells. The DRG cells were replaced by mesodermal/connective tissue. In the remaining patients, the cellular units were well preserved. Preoperative plexus MRI neurography was not able to distinguish the two types. Patients with 'neuronal preservation' had less maximum pain than patients with 'neuronal loss'. Arm function improved after nerve reconstruction, but severe pain persisted. Transcriptome analysis of preserved DRGs revealed expression of subtype-specific sensory neurone marker genes, but downregulation of neuronal attributes. Furthermore, they showed signs of ongoing inflammation and connective tissue remodelling.

CONCLUSIONS

Patients with plexus injury separate into two groups with either neuronal preservation or neuronal loss. The former could benefit from anti-inflammatory therapy. For the latter, studies should explore mechanisms of neuronal loss especially for regenerative approaches.

CLINICAL TRIAL REGISTRATION

DRKS00017266.

Decoding the regulatory role of ATP synthase inhibitory factor 1 (ATPIF1) in Wallerian degeneration and peripheral nerve regeneration

ATP synthase inhibitory factor 1 (ATPIF1), a key modulator of ATP synthase complex activity, has been implicated in various physiological and pathological processes. While its role is established in conditions such as hypoxia, ischemia-reperfusion injury, apoptosis, and cancer, its involvement remains elusive in peripheral nerve regeneration. Leveraging ATPIF1 knockout transgenic mice, this study reveals that the absence of ATPIF1 impedes neural structural reconstruction, leading to delayed sensory and functional recovery. RNA-sequencing unveils a significant attenuation in immune responses following peripheral nerve injury, which attributes to the CCR2/CCL2 signaling axis and results in decreased macrophage infiltration and activation. Importantly, macrophages, not Schwann cells, are identified as key contributors to the delayed Wallerian degeneration in ATPIF1 knockout mice, and affect the overall outcome of peripheral nerve regeneration. These results shed light on the translational potential of ATPIF1 for improving peripheral nerve regeneration.

Dorsal root ganglia CSF1+ neuronal subtypes have different impact on macrophages and microglia after spared nerve injury

BACKGROUND AND AIMS

Colony-stimulating factor 1 (CSF1) is a growth factor secreted by dorsal root ganglia (DRG) neurons important for DRG macrophages and spinal cord (SC) microglia injury-induced proliferation and activation, specifically released after spared nerve injury (SNI). In this study, we investigated if SNI-induced CSF1 expression and perineuronal rings of macrophages around mouse DRG neurons vary between L3-L5 DRG and with the neuronal type, and if the CSF1+ neuronal projections at the SC dorsal horns were associated with an increased microglial number in the corresponding laminae.

METHODS

Seven days after surgery, L3-L5 DRG as well as their corresponding segments at the SC level were collected, frozen, and cut. DRG sections were double-immunostained using antibodies against CSF1 and NF200, CGRP or IB4, while SC sections were immunostained using a fluorescent Nissl Stain and analyzed for CX3CR1-GFP microglia number and distribution by an in-house ImageJ Plug-in.

RESULTS

Our results showed that SNI-induced CSF1 expression was common for all subtypes of mouse DRG neurons, being responsible for attracting more resident macrophages around them in a DRG-dependent manner, with L4 showing the stronger response and CSF1+/NF200+ neurons showing the highest incidence. Even though the total number of microglia in the SC ipsilateral dorsal horns increased after SNI, the increase at their specific laminar projection sites did not mirror the incidence of DRG neuronal subtypes among CSF1+ neurons.

INTERPRETATION

Taken together, these results contribute to a more comprehensive understanding of the connection between CSF1 and macrophage/microglia response after SNI and emphasize the importance of considering L3-L5 DRG individually when investigating SNI-neuropathic pain pathogenesis in mice.

Promotive effect of skin precursor-derived Schwann cells on brachial plexus neurotomy and motor neuron damage repair through milieu-regulating secretome

Brachial plexus injury (BPI) with motor neurons (MNs) damage still remain poor recovery in preclinical research and clinical therapy, while cell-based therapy approaches emerged as novel strategies. Previous work of rat skin precursor-derived Schwann cells (SKP-SCs) provided substantial foundation for repairing peripheral nerve injury (PNI). Given that, our present work focused on exploring the repair efficacy and possible mechanisms of SKP-SCs implantation on rat BPI combined with neurorrhaphy post-neurotomy. Results indicated the significant locomotive and sensory function recovery, with improved morphological remodeling of regenerated nerves and angiogenesis, as well as amelioration of target muscles atrophy and motor endplate degeneration. Besides, MNs could restore from oxygen-glucose-deprivation (OGD) injury upon SKP-SCs-sourced secretome treatment, implying the underlying paracrine mechanisms. Moreover, rat cytokine array assay detected 67 cytokines from SKP-SC-secretome, and bioinformatic analyses of screened 32 cytokines presented multiple functional clusters covering diverse cell types, including inflammatory cells, Schwann cells, vascular endothelial cells (VECs), neurons, and SKP-SCs themselves, relating distinct biological processes to nerve regeneration. Especially, a panel of hypoxia-responsive cytokines (HRCK), can participate into multicellular biological process regulation for permissive regeneration milieu, which underscored the benefits of SKP-SCs and sourced secretome, facilitating the chorus of nerve regenerative microenvironment. Furthermore, platelet-derived growth factor-AA (PDGF-AA) and vascular endothelial growth factor-A (VEGF-A) were outstanding cytokines involved with nerve regenerative microenvironment regulating, with significantly elevated mRNA expression level in hypoxia-responsive SKP-SCs. Altogether, through recapitulating the implanted SKP-SCs and derived secretome as niche sensor and paracrine transmitters respectively, HRCK would be further excavated as molecular underpinning of the neural recuperative mechanizations for efficient cell therapy; meanwhile, the analysis paradigm in this study validated and anticipated the actions and mechanisms of SKP-SCs on traumatic BPI repair, and was beneficial to identify promising bioactive molecule cocktail and signaling targets for cell-free therapy strategy on neural repair and regeneration.

Schwann cell-secreted frizzled-related protein 1 dictates neuroinflammation and peripheral nerve degeneration after neurotrauma

Neurotrauma in limbs can induce sustained neuroinflammation, resulting in persistent disruption of nerve tissue architecture and retardation of axon regrowth. Despite macrophage-mediated inflammation promoting the removal of necrotic neural components and stimulating neo-vessel ingrowth, detrimental shifts in macrophage phenotype exacerbate nerve degeneration. Herein, we find that peripheral nerve injuries (PNIs) result in abundant secreted frizzled-related protein 1 (sFRP1) expression, particularly by Schwann cells (SCs). Heat shock protein 90 (HSP90) in macrophages recognizes sFRP1 and triggers a dysregulated secretion of inflammatory mediators. Single-cell atlas of human injured peripheral nerves reveals the appearance of sFRP1-expressing SCs with mesenchymal traits and macrophages with a proinflammatory genetic profile. Deletion of either SC-specific sFRP1 or macrophage-specific HSP90 alleviates neuroinflammation and prevents the progression of nerve degeneration. Together, our findings implicate the response of macrophages to SC-derived sFRP1 in exacerbating nerve damage following PNIs.

Differential regulation of tissue-resident and blood-derived macrophages in models of autoimmune and traumatic peripheral nerve injury

Introduction

The current study focuses on understanding the functional role of different subsets of endoneurial macrophages in autoimmune polyneuropathies (AP) and traumatic peripheral nerve injury (TPNI), which holds potential for clinical application. Recent studies have advanced our understanding of the diverse origins of macrophages within peripheral nerves. However, there remains a gap in our knowledge regarding how endoneurial macrophages from different origins affect disease progression in AP versus TPNI.

Methods

Flow cytometry was utilized to analyze macrophage phenotypes, including polarization states, cytokine production, and myelin phagocytosis in animal models of AP and TPNI. This study focuses on two distinct origins of macrophages, namely CD11b+F4/80hi tissue-resident (TRM) and CD11b+F4/80int blood-derived macrophages (BDM). The study utilized two animal models: the first was the spontaneous autoimmune peripheral polyneuropathy (SAPP) model in B7.2-null non-obese diabetic (NOD-B7.2-/-) mice, which serves as a model for inflammatory demyelinating polyneuropathy; the second model involved wild type C57BL/6 mice subjected to sciatic nerve crush injury, modeling TPNI. Behavioral, electrophysiological, and histological analyses were performed to assess peripheral nerve injury.

Results

The study found that pro-inflammatory M1 macrophage polarization and tumor necrosis factor-alpha production by macrophages were more pronounced in the peripheral nerves of SAPP mice compared to those with TPNI, with the majority of these macrophages being TRM. In contrast, endoneurial macrophages in mice with TPNI were mainly BDM, exhibiting a less defined macrophage polarization and cytokine profile than TRM in AP mice. Interestingly, myelin phagocytosis was primarily driven by BDM in both SAPP and TPNI mice.

Discussion

This study offers novel insights into origin-dependent macrophage functions in AP and TPNI. Furthermore, these findings may help the future development of novel therapies targeting macrophage subsets of specific origin in AP and TPNI.

Single-cell profiling of cellular changes in the somatic peripheral nerves following nerve injury

Injury to the peripheral nervous system disconnects targets to the central nervous system, disrupts signal transmission, and results in functional disability. Although surgical and therapeutic treatments improve nerve regeneration, it is generally hard to achieve fully functional recovery after severe peripheral nerve injury. A better understanding of pathological changes after peripheral nerve injury helps the development of promising treatments for nerve regeneration. Single-cell analyses of the peripheral nervous system under physiological and injury conditions define the diversity of cells in peripheral nerves and reveal cell-specific injury responses. Herein, we review recent findings on the single-cell transcriptome status in the dorsal root ganglia and peripheral nerves following peripheral nerve injury, identify the cell heterogeneity of peripheral nerves, and delineate changes in injured peripheral nerves, especially molecular changes in neurons, glial cells, and immune cells. Cell-cell interactions in peripheral nerves are also characterized based on ligand-receptor pairs from coordinated gene expressions. The understanding of cellular changes following peripheral nerve injury at a single-cell resolution offers a comprehensive and insightful view for the peripheral nerve repair process, provides an important basis for the exploration of the key regulators of neuronal growth and microenvironment reconstruction, and benefits the development of novel therapeutic drugs for the treatment of peripheral nerve injury.

The protective effect of erythropoietin and its novel derived peptides in peripheral nerve injury

Peripheral nerve injury seriously endangers human life and health, but there is no clinical drug for the treatment of peripheral nerve injury, so it is imperative to develop drugs to promote the repair of peripheral nerve injury. Erythropoietin (EPO) not only has the traditional role of promoting erythropoiesis, but also has a tissue-protective effect. Over the past few decades, researchers have confirmed that EPO has neuroprotective effects. However, side effects caused by long-term use of EPO limited its clinical application. Therefore, EPO derivatives with low side effects have been explored. Among them, ARA290 has shown significant protective effects on the nervous system, but the biggest disadvantage of ARA290, its short half-life, limits its application. To address the short half-life issue, the researchers modified ARA290 with thioether cyclization to generate a thioether cyclized helical B peptide (CHBP). ARA290 and CHBP have promising applications as peptide drugs. The neuroprotective effects they exhibit have attracted continuous exploration of their mechanisms of action. This article will review the research on the role of EPO, ARA290 and CHBP in the nervous system around this developmental process, and provide a certain reference for the subsequent research.

Activation of the α7nAChR by GTS-21 mitigates septic tubular cell injury and modulates macrophage infiltration

The most common and serious complication among hospitalized and critically ill patients is sepsis-associated acute kidney damage (S-AKI), which raises the risk of comorbidities and is linked to a high mortality rate. Cholinergic anti-inflammatory pathway (CAP), an anti-inflammatory pathway mediated by the vagus nerve, acetylcholine, and α7 nicotinic acetylcholine receptors (α7nAChRs), offers new perspectives for the treatment of S-AKI. In this study, we investigated the role of CAP and α7nAChR in kidney injury by employing an LPS-induced septic kidney injury mouse model and GTS-21 intervention. C57BL/6 mice were injected with LPS, with or without GTS-21, in different subgroups. Kidney function was assessed by plasma creatinine, histology, and markers of kidney injury 24 h after intervention. The results demonstrated that GTS-21 could inhibit the systemic inflammatory response and directly protect the tubular cell injury from LPS. To explore the novel gene involved in this response, RNA sequencing of the renal proximal tubular epithelial cell (HK-2), pretreated with LPS and GTS-21, was conducted. The results indicate that GTS-21 administration reduces LPS-induced cytokines and chemokines secretion by HK-2, including CCL20, a potent chemokine attracting monocytes/macrophages. Furthermore, a macrophage transmigration assay revealed that GTS-21 inhibits macrophage transmigration by downregulating the expression of CCL20 in HK-2 cells. In conclusion, GTS-21, as an α7nAChR agonist, emerges as a noteworthy and versatile treatment for S-AKI. Its dual function of directly protecting renal tubular cells and regulating inflammatory responses represents a major advancement in the treatment of sepsis-induced AKI. This finding might pave the way for novel approaches to improving patient outcomes and reducing death rates in sepsis-related complications.

The vicious cycle of chronic endometriosis and depression-an immunological and physiological perspective

Endometriosis is a chronic, estrogen-dependent, proinflammatory disease that can cause various dysfunctions. The main clinical manifestations of endometriosis include chronic pelvic pain and impaired fertility. The disease is characterized by a spectrum of dysfunctions spanning hormonal signaling, inflammation, immune dysregulation, angiogenesis, neurogenic inflammation, epigenetic alterations, and tissue remodeling. Dysregulated hormonal signaling, particularly involving estrogen and progesterone, drives abnormal growth and survival of endometrial-like tissue outside the uterus. Chronic inflammation, marked by immune cell infiltration and inflammatory mediator secretion, perpetuates tissue damage and pain. Altered immune function, impaired ectopic tissue clearance, and dysregulated cytokine production contribute to immune dysregulation. Enhanced angiogenesis promotes lesion growth and survival. Epigenetic modifications influence gene expression patterns, e.g., HSD11B1 gene, affecting disease pathogenesis. Endometriosis related changes and infertility lead to depression in diagnosed women. Depression changes lifestyle and induces physiological and immunological changes. A higher rate of depression and anxiety has been reported in women diagnosed with endometriosis, unleashing physiological, clinical and immune imbalances which further accelerate chronic endometriosis or vice versa. Thus, both endometriosis and depression are concomitantly part of a vicious cycle that enhance disease complications. A multidimensional treatment strategy is needed which can cater for both endometrial disease and depression and anxiety disorders.

Cytoarchitecture and intercellular interactions in the trigeminal ganglion: Associations with neuropathic pain in the orofacial region

BACKGROUND

Disorders of the trigeminal nerve, a sensory nerve of the orofacial region, often lead to complications in dental practice, including neuropathic pain, allodynia, and ectopic pain. Management of these complications requires an understanding of the cytoarchitecture of the trigeminal ganglion, where the cell bodies of the trigeminal nerve are located, and the mechanisms of cell-cell interactions.

HIGHLIGHTS

In the trigeminal ganglion, ganglion, satellite, Schwann, and immune cells coexist and interact. Cell-cell interactions are complex and occur through direct contact via gap junctions or through mediators such as adenosine triphosphate, nitric oxide, peptides, and cytokines. Interactions between the nervous and immune systems within the trigeminal ganglion may have neuroprotective effects during nerve injury or may exacerbate inflammation and produce chronic pain. Under pathological conditions of the trigeminal nerve, cell-cell interactions can cause allodynia and ectopic pain. Although cell-cell interactions that occur via mediators can act at some distance, they are more effective when the cells are close together. Therefore, information on the three-dimensional topography of trigeminal ganglion cells is essential for understanding the pathophysiology of ectopic pain.

CONCLUSIONS

A three-dimensional map of the somatotopic localization of trigeminal ganglion neurons revealed that ganglion cells innervating distant orofacial regions are often apposed to each other, interacting with and potentially contributing to ectopic pain. Elucidation of the complex network of mediators and their receptors responsible for intercellular communication within the trigeminal ganglion is essential for understanding ectopic pain.

Exogenous NT-3 Promotes Phenotype Switch of Resident Macrophages and Improves Sciatic Nerve Injury through AMPK/NF-κB Signaling Pathway

Neurotrophin-3 (NT-3) is an important family of neurotrophic factors with extensive neurotrophic activity, which can maintain the survival and regeneration of nerve cells. However, the mechanism of NT-3 on macrophage phenotype transformation after sciatic nerve injury is not clear. In this study, we constructed a scientific nerve compression injury animal model and administered different doses of NT-3 treatment through osmotic minipump. 7 days after surgery, we collected sciatic nerve tissue and observed the distribution of macrophage phenotype through iNOS and CD206 immunofluorescence. During the experiment, regular postoperative observations were conducted on rats. After the experiment, sciatic nerve tissue was collected for HE staining, myelin staining, immunofluorescence staining, and Western blot analysis. To verify the role of the AMPK/NF-κB pathway, we applied the AMPK inhibitor Compound C and the NF-κB inhibitor BAY11-7082 to repeat the above experiment. Our experimental results reveal that NT-3 promotes sciatic nerve injury repair and polarization of M2 macrophage phenotype, promotes AMPK activation, and inhibits NF-κB activation. The repair effect of high concentration NT-3 on sciatic nerve injury is significantly enhanced compared to low concentration. Compound C administration can weaken the effect of NT-3, while BAY 11-7082 can enhance the effect of NT-3. In short, NT-3 significantly improves sciatic nerve injury in rats, promotes sciatic nerve function repair, accelerates M2 macrophage phenotype polarization, and improves neuroinflammatory response. The protective effects of NT-3 mentioned above are partially related to the AMPK/NF-κB signal axis.

Macrophages of multiple hematopoietic origins reside in the developing prostate

Tissue-resident macrophages contribute to the organogenesis of many tissues. Growth of the prostate is regulated by androgens during puberty, yet androgens are considered immune suppressive. In this study, we characterized the localization, androgen receptor expression and hematopoietic origin of prostate macrophages, and transiently ablated macrophages during postnatal prostate organogenesis in the mouse. We show that myeloid cells were abundant in the prostate during puberty. However, nuclear androgen receptor expression was not detected in most macrophages. We found Cx3cr1, a marker for macrophages, monocytes and dendritic cells, expressed in interstitial macrophages surrounding the prostate and associated with nerve fibers. Furthermore, we provide evidence for the co-existence of embryonic origin, self-renewing, tissue-resident macrophages and recruited macrophages of bone-marrow monocyte origin in the prostate during puberty. Our findings suggest that prostate macrophages promote neural patterning and may shed further light on our understanding of the role of the innate immune system in prostate pathology in response to inflammation and in cancer.

Sciatic nerve stimulation alleviates neuropathic pain and associated neuroinflammation in the dorsal root ganglia in a rodent model

BACKGROUND

Satellite glial cells (SGCs) in the dorsal root ganglia (DRG) play a pivotal role in the formation of neuropathic pain (NP). Sciatic nerve stimulation (SNS) neuromodulation was reported to alleviate NP and reduce neuroinflammation. However, the mechanisms underlying SNS in the DRG remain unclear. This study aimed to elucidate the mechanism of electric stimulation in reducing NP, focusing on the DRG.

METHODS

L5 nerve root ligation (NRL) NP rat model was studied. Ipsilateral SNS performed 1 day after NRL. Behavioral tests were performed to assess pain phenotypes. NanoString Ncounter technology was used to explore the differentially expressed genes and cellular pathways. Activated SGCs were characterized in vivo and in vitro. The histochemical alterations of SGCs, macrophages, and neurons in DRG were examined in vivo on post-injury day 8.

RESULTS

NRL induced NP behaviors including decreased pain threshold and latency on von Frey and Hargreaves tests. We found that following nerve injury, SGCs were hyperactivated, neurotoxic and had increased expression of NP-related ion channels including TRPA1, Cx43, and SGC-neuron gap junctions. Mechanistically, nerve injury induced reciprocal activation of SGCs and M1 macrophages via cytokines including IL-6, CCL3, and TNF-α mediated by the HIF-1α-NF-κB pathways. SNS suppressed SGC hyperactivation, reduced the expression of NP-related ion channels, and induced M2 macrophage polarization, thereby alleviating NP and associated neuroinflammation in the DRG.

CONCLUSIONS

NRL induced hyperactivation of SGCs, which had increased expression of NP-related ion channels. Reciprocal activation of SGCs and M1 macrophages surrounding the primary sensory neurons was mediated by the HIF-1α and NF-κB pathways. SNS suppressed SGC hyperactivation and skewed M1 macrophage towards M2. Our findings establish SGC activation as a crucial pathomechanism in the gliopathic alterations in NP, which can be modulated by SNS neuromodulation.

GDPD3 Deficiency Alleviates Neuropathic Pain and Reprograms Macrophagic Polarization Through PGE2 and PPARγ Pathway

The complex mechanism of neuropathic pain involves various aspects of both central and peripheral pain conduction pathways. An effective cure for neuropathic pain therefore remains elusive. We found that deficiency of the gene Gdpd3, encoding a lysophospholipase D enzyme, alleviates the inflammatory responses in dorsal root ganglia (DRG) of mice under neuropathic pain and reduces PE (20:4) and PGE2 in DRG. Gdpd3 deficiency had a stronger analgesic effect on neuropathic pain than Celecoxib, a nonsteroidal anti-inflammatory drug. Gdpd3 deficiency also interferes with the polarization of macrophages, switching from M1 towards M2 phenotype. The PPARγ/ FABP4 pathway was screened by RNA sequencing as functional related with Gdpd3 deficient BMDMs stimulated with LPS. Both protein and mRNA levels of PPARγ in GDPD3 deficient BMDMs were higher than those of the litter control mice. However, GW9962 (inhibitor of PPARγ) could reverse the reprogramming polarization of macrophages caused by GDPD3 deficiency. Therefore, our study suggests that GDPD3 deficiency exerts a relieving effect on neuropathic pain and alleviates neuroinflammation in DRG by switching the phenotype of macrophages from M1 to M2, which was mediated through PGE2 and PPARγ/ FABP4 pathway.

Interpretation of Data from Translational Rodent Nerve Injury and Repair Models

This article highlights the use of rodents as preclinical models to evaluate the management of nerve injuries, describing the pitfalls and value from rodent nerve injury and regeneration outcomes, as well as treatments derived from these rodent models. The anatomic structure, size, and cellular and molecular differences and similarities between rodent and human nerves are summarized. Specific examples of success and failure when assessing outcome metrics are presented for context. Evidence for translation to clinical practice includes the topics of electrical stimulation, Tacrolimus (FK506), and acellular nerve allografts.

Neutrophil peptide 1 accelerates the clearance of degenerative axons during Wallerian degeneration by activating macrophages after peripheral nerve crush injury

JOURNAL/nrgr/04.03/01300535-202408000-00036/figure1/v/2023-12-16T180322Z/r/image-tiff Macrophages play an important role in peripheral nerve regeneration, but the specific mechanism of regeneration is still unclear. Our preliminary findings indicated that neutrophil peptide 1 is an innate immune peptide closely involved in peripheral nerve regeneration. However, the mechanism by which neutrophil peptide 1 enhances nerve regeneration remains unclear. This study was designed to investigate the relationship between neutrophil peptide 1 and macrophages in vivo and in vitro in peripheral nerve crush injury. The functions of RAW 264.7 cells were elucidated by Cell Counting Kit-8 assay, flow cytometry, migration assays, phagocytosis assays, immunohistochemistry and enzyme-linked immunosorbent assay. Axonal debris phagocytosis was observed using the CUBIC (Clear, Unobstructed Brain/Body Imaging Cocktails and Computational analysis) optical clearing technique during Wallerian degeneration. Macrophage inflammatory factor expression in different polarization states was detected using a protein chip. The results showed that neutrophil peptide 1 promoted the proliferation, migration and phagocytosis of macrophages, and CD206 expression on the surface of macrophages, indicating M2 polarization. The axonal debris clearance rate during Wallerian degeneration was enhanced after neutrophil peptide 1 intervention. Neutrophil peptide 1 also downregulated inflammatory factors interleukin-1α, -6, -12, and tumor necrosis factor-α in vivo and in vitro. Thus, the results suggest that neutrophil peptide 1 activates macrophages and accelerates Wallerian degeneration, which may be one mechanism by which neutrophil peptide 1 enhances peripheral nerve regeneration.

RNA sequencing of exosomes secreted by fibroblast and Schwann cells elucidates mechanisms underlying peripheral nerve regeneration

JOURNAL/nrgr/04.03/01300535-202408000-00035/figure1/v/2023-12-16T180322Z/r/image-tiff Exosomes exhibit complex biological functions and mediate a variety of biological processes, such as promoting axonal regeneration and functional recovery after injury. Long non-coding RNAs (lncRNAs) have been reported to play a crucial role in axonal regeneration. However, the role of the lncRNA-microRNA-messenger RNA (mRNA)-competitive endogenous RNA (ceRNA) network in exosome-mediated axonal regeneration remains unclear. In this study, we performed RNA transcriptome sequencing analysis to assess mRNA expression patterns in exosomes produced by cultured fibroblasts (FC-EXOs) and Schwann cells (SC-EXOs). Differential gene expression analysis, Gene Ontology analysis, Kyoto Encyclopedia of Genes and Genomes analysis, and protein-protein interaction network analysis were used to explore the functions and related pathways of RNAs isolated from FC-EXOs and SC-EXOs. We found that the ribosome-related central gene Rps5 was enriched in FC-EXOs and SC-EXOs, which suggests that it may promote axonal regeneration. In addition, using the miRWalk and Starbase prediction databases, we constructed a regulatory network of ceRNAs targeting Rps5, including 27 microRNAs and five lncRNAs. The ceRNA regulatory network, which included Ftx and Miat, revealed that exsosome-derived Rps5 inhibits scar formation and promotes axonal regeneration and functional recovery after nerve injury. Our findings suggest that exosomes derived from fibroblast and Schwann cells could be used to treat injuries of peripheral nervous system.

Role of M1/M2 macrophages in pain modulation

Pain is a signal of inflammation that can have both protective and pathogenic effects. Macrophages, significant components of the immune system, play crucial roles in the occurrence and development of pain, particularly in neuroimmune communication. Macrophages exhibit plasticity and heterogeneity, adopting either pro-inflammatory M1 or anti-inflammatory M2 phenotypes depending on their functional orientation. Recent research highlights the contribution of macrophages to pain dynamics by undergoing changes in their functional polarity, leading to macrophage activation, tissue infiltration, and cytokine secretion. M1 macrophages release pro-inflammatory mediators that are not only essential in defending against infections, but also contributing to tissue damage and the elicitation of pain. However, this process can be counteracted by M2 macrophages, facilitating pain relief through producing anti-inflammatory cytokines and opioid peptides or enhancing efferocytosis. M1 and M2 macrophages play important roles in both the initiation and mitigation of pain.

Exosomes repairment for sciatic nerve injury: a cell-free therapy

Sciatic nerve injury (SNI) is a common type of peripheral nerve injury typically resulting from trauma, such as contusion, sharp force injuries, drug injections, pelvic fractures, or hip dislocations. It leads to both sensory and motor dysfunctions, characterized by pain, numbness, loss of sensation, muscle atrophy, reduced muscle tone, and limb paralysis. These symptoms can significantly diminish a patient's quality of life. Following SNI, Wallerian degeneration occurs, which activates various signaling pathways, inflammatory factors, and epigenetic regulators. Despite the availability of several surgical and nonsurgical treatments, their effectiveness remains suboptimal. Exosomes are extracellular vesicles with diameters ranging from 30 to 150 nm, originating from the endoplasmic reticulum. They play a crucial role in facilitating intercellular communication and have emerged as highly promising vehicles for drug delivery. Increasing evidence supports the significant potential of exosomes in repairing SNI. This review delves into the pathological progression of SNI, techniques for generating exosomes, the molecular mechanisms behind SNI recovery with exosomes, the effectiveness of combining exosomes with other approaches for SNI repair, and the changes and future outlook for utilizing exosomes in SNI recovery.

Quantitative mass spectrometry analysis of the injured proximal and distal human digital nerve ends

Introduction

Proteomic analysis of injured human peripheral nerves, particularly focusing on events occurring in the proximal and distal nerve ends, remains relatively underexplored. This study aimed to investigate the molecular patterns underlying a digital nerve injury, focusing on differences in protein expression between the proximal and distal nerve ends.

Methods

A total of 26 human injured digital nerve samples (24 men; 2 women; median age 47 [30-66] years), harvested during primary nerve repair within 48 h post-injury from proximal and distal nerve ends, were analyzed using mass spectrometry.

Results

A total of 3,914 proteins were identified, with 127 proteins showing significant differences in abundance between the proximal and the distal nerve ends. The downregulation of proteins in the distal nerve end was associated with synaptic transmission, autophagy, neurotransmitter regulation, cell adhesion and migration. Conversely, proteins upregulated in the distal nerve end were implicated in cellular stress response, neuromuscular junction stability and muscle contraction, neuronal excitability and neurotransmitter release, synaptic vesicle recycling and axon guidance and angiogenesis.

Discussion

Investigation of proteins, with functional annotations analysis, in proximal and the distal ends of human injured digital nerves, revealed dynamic cellular responses aimed at promoting tissue degeneration and restoration, while suppressing non-essential processes.

Berberine inhibits NLRP3 inflammasome activation and proinflammatory macrophage M1 polarization to accelerate peripheral nerve regeneration

Berberine (BBR) has demonstrated potent anti-inflammatory effects by modulating macrophage polarization. Nevertheless, the precise mechanisms through which berberine regulates post-injury inflammation within the peripheral nerve system remain elusive. This study seeks to elucidate the role of BBR and its underlying mechanisms in inflammation following peripheral nerve injury (PNI). Adult male C57BL/6J mice subjected to PNI were administered daily doses of berberine (0, 60, 120, 180, 240 ​mg/kg) via gavage from day 1 through day 28. Evaluation of the sciatic function index (SFI) and paw withdrawal threshold revealed that BBR dose-dependently enhanced both motor and sensory functions. Immunofluorescent staining for anti-myelin basic protein (anti-MBP) and anti-neurofilament-200 (anti-NF-200), along with histological staining comprising hematoxylin-eosin (HE), luxol fast blue (LFB), and Masson staining, demonstrated that BBR dose-dependently promoted structural regeneration. Molecular analyses including qRT-PCR, Western blotting, enzyme-linked immunosorbent assay (ELISA), and immunofluorescence confirmed that inactivation of the NLRP3 inflammasome by MCC950 shifted macrophages from the pro-inflammatory M1 phenotype to the anti-inflammatory M2 phenotype, while also impeding macrophage infiltration. Furthermore, BBR significantly downregulated the expression of the NLRP3 inflammasome and its associated molecules in macrophages, thereby mitigating NLRP3 inflammasome activation-induced macrophage M1 polarization and inflammation. In summary, BBR's neuroprotective effects were concomitant with the suppression of inflammation after PNI, achieved through the inhibition of NLRP3 inflammasome activation-induced macrophage M1 polarization.

Intravenous application of human umbilical cord mesenchymal stem cells alleviate neuropathic pain by suppressing microglia activation in rats

Objective

Neuropathic pain has been considered as one of the most serious chronic pain subtypes and causes intolerable suffering to patients physically and mentally. This study aimed to verify the analgesic effect of intravenous administration of human umbilical cord mesenchymal stem cells (HUC-MSCs) upon rats with chronic constriction injury (CCI)-induced neuropathic pain and the concomitant mechanism via modulating microglia.

Methods

30 male SD rats were randomized divided into three groups (n = 10 per group): Sham + Saline group (S&S group), CCI + Saline group (C&S group) and CCI + HUC-MSCs group (C&U group). Rats were injected with either saline or HUC-MSCs via the caudal vein on the 7th day after modelling. The paw mechanical withdrawal threshold (PMWT) and thermal withdrawal latency (TWL) of the ligation side were measured before (day 0) and after (day 1, 3, 5, 7, 9, 11, 13, and 15) modelling. On day 15 after modelling, western-blotting and immunofluorescent staining were used to assess the expressive abundance of Iba-1 (a typical biomarker of activated microglia) in the ligation side of the spinal cord dorsal horn, and ultrastructural changes of the ligation of sciatic nerve were evaluated by transmission electron microscope (TEM).

Results

Compared with the S&S group, PMWT and TWL in the C&S group were significantly decreased on day 5 and then persisted to day 15 after modelling (C&S vs S&S, P < 0.05), while a significant amelioration of mechanical hyperalgesia (day 13, day 15) and thermal allodynia (day 9, day 11, day 15) was observed in the C&U group (C&U vs C&S, P < 0.05). Meanwhile, the expression of Iba-1 was significantly suppressed by systemic infusion of HUC-MSCs in the C&U group according to western-blotting and immunofluorescent staining analyses (P < 0.05). With the aid of TEM detection, we intuitively noticed the efficacious reconstruction of the laminate structure of the sciatic nerve ligation, elimination of mitochondrial swelling, and formation of new myelination were noted on day 15 after modelling in the C&U group.

Conclusions

Overall, intravenous administration of HUC-MSCs systemically revealed an ameliorative effect upon CCI-induced neuropathic pain in SD rats by inhibiting microglia activation in the dorsal horn of the impaired spinal cord and alleviating sciatic nerve injury. Our findings supply new references for the further development of HUC-MSCs-based cytotherapy for neuropathic pain administration.

Electrical stimulation with polypyrrole-coated polycaprolactone/silk fibroin scaffold promotes sacral nerve regeneration by modulating macrophage polarisation

Peripheral nerve injury poses a great threat to neurosurgery and limits the regenerative potential of sacral nerves in the neurogenic bladder. It remains unknown whether electrical stimulation can facilitate sacral nerve regeneration in addition to modulate bladder function. The objective of this study was to utilise electrical stimulation in sacra nerve crush injury with newly constructed electroconductive scaffold and explore the role of macrophages in electrical stimulation with crushed nerves. As a result, we generated a polypyrrole-coated polycaprolactone/silk fibroin scaffold through which we applied electrical stimulation. The electrical stimulation boosted nerve regeneration and polarised the macrophages towards the M2 phenotype. An in vitro test using bone marrow derived macrophages revealed that the pro-regenerative polarisation of M2 were significantly enhanced by electrical stimulation. Bioinformatics analysis showed that the expression of signal transducer and activator of transcriptions (STATs) was differentially regulated in a way that promoted M2-related genes expression. Our work indicated the feasibility of electricals stimulation used for sacral nerve regeneration and provided a firm demonstration of a pivotal role which macrophages played in electrical stimulation.

OTULIN of exosomes derived from Schwann cells promotes peripheral nerve injury repair by regulating macrophage polarization via deubiquitination of ERBB2

A significant public health burden is peripheral nerve damage (PNI), which is frequently brought on by trauma. Macrophages were essential to the effective regeneration of nerves and restoration of function. It is still not entirely understood how macrophages and Schwann cells interact after damage during remyelination. Here, we established an inflammatory model in bone marrow-derived macrophages (BMDMs) and a rat sciatic nerve damage model to investigate the possible relationship between lipopolysaccharides (LPS)-induced exosomes derived from Schwann cells (LPS SCs-Exos) and peripheral nerve repair. The pro-inflammatory macrophage was changed into a pro-regeneration macrophage by LPS SC-Exos. Notably, it was discovered that SC-Exos had a substantial enrichment of OTULIN. OTULIN was a key mediator in the regulatory effects of LPS SC-Exos by deubiquitinating ERBB2 and preventing its degradation. The local injection of SC-Exos into the nerve damage site led in a faster functional recovery, axon regeneration and remyelination, and an increased M2 macrophage polarization, whereas OTULIN knockdown reversed these effects in vivo. Our results indicate that LPS SC-Exos may offer a therapeutic avenue for peripheral nerve regeneration by promoting macrophage polarization toward an M2 phenotype through the shuttling of OTULIN and deubiquitination of ERBB2. SIGNIFICANCE STATEMENT: OTULIN protein from SC-Exos mediated the macrophages polarization and axonal growth in BMDMs through promoting ubiquitination of ERBB2 and triggering the degradation of ERBB2. The findings offered prospective therapeutic hints for PNI therapy approaches that target axonal regrowth.

Limited Nerve Regeneration across Acellular Nerve Allografts (ANAs) Coincides with Changes in Blood Vessel Morphology and the Development of a Pro-Inflammatory Microenvironment

The use of acellular nerve allografts (ANAs) to reconstruct long nerve gaps (>3 cm) is associated with limited axon regeneration. To understand why ANA length might limit regeneration, we focused on identifying differences in the regenerative and vascular microenvironment that develop within ANAs based on their length. A rat sciatic nerve gap model was repaired with either short (2 cm) or long (4 cm) ANAs, and histomorphometry was used to measure myelinated axon regeneration and blood vessel morphology at various timepoints (2-, 4- and 8-weeks). Both groups demonstrated robust axonal regeneration within the proximal graft region, which continued across the mid-distal graft of short ANAs as time progressed. By 8 weeks, long ANAs had limited regeneration across the ANA and into the distal nerve (98 vs. 7583 axons in short ANAs). Interestingly, blood vessels within the mid-distal graft of long ANAs underwent morphological changes characteristic of an inflammatory pathology by 8 weeks post surgery. Gene expression analysis revealed an increased expression of pro-inflammatory cytokines within the mid-distal graft region of long vs. short ANAs, which coincided with pathological changes in blood vessels. Our data show evidence of limited axonal regeneration and the development of a pro-inflammatory environment within long ANAs.

Plasma membrane abundance dictates phagocytic capacity and functional cross-talk in myeloid cells

Professional phagocytes like neutrophils and macrophages tightly control what they consume, how much they consume, and when they move after cargo uptake. We show that plasma membrane abundance is a key arbiter of these cellular behaviors. Neutrophils and macrophages lacking the G protein subunit Gβ4 exhibited profound plasma membrane expansion, accompanied by marked reduction in plasma membrane tension. These biophysical changes promoted the phagocytosis of bacteria, fungus, apoptotic corpses, and cancer cells. We also found that Gβ4-deficient neutrophils are defective in the normal inhibition of migration following cargo uptake. Sphingolipid synthesis played a central role in these phenotypes by driving plasma membrane accumulation in cells lacking Gβ4. In Gβ4 knockout mice, neutrophils not only exhibited enhanced phagocytosis of inhaled fungal conidia in the lung but also increased trafficking of engulfed pathogens to other organs. Together, these results reveal an unexpected, biophysical control mechanism central to myeloid functional decision-making.

Inflammation in the Peripheral Nervous System after Injury

Nerve injury is a common condition that occurs as a result of trauma, iatrogenic injury, or long-lasting stimulation. Unlike the central nervous system (CNS), the peripheral nervous system (PNS) has a strong capacity for self-repair and regeneration. Peripheral nerve injury results in the degeneration of distal axons and myelin sheaths. Macrophages and Schwann cells (SCs) can phagocytose damaged cells. Wallerian degeneration (WD) makes the whole axon structure degenerate, creating a favorable regenerative environment for new axons. After nerve injury, macrophages, neutrophils and other cells are mobilized and recruited to the injury site to phagocytose necrotic cells and myelin debris. Pro-inflammatory and anti-inflammatory factors involved in the inflammatory response provide a favorable microenvironment for peripheral nerve regeneration and regulate the effects of inflammation on the body through relevant signaling pathways. Previously, inflammation was thought to be detrimental to the body, but further research has shown that appropriate inflammation promotes nerve regeneration, axon regeneration, and myelin formation. On the contrary, excessive inflammation can cause nerve tissue damage and pathological changes, and even lead to neurological diseases. Therefore, after nerve injury, various cells in the body interact with cytokines and chemokines to promote peripheral nerve repair and regeneration by inhibiting the negative effects of inflammation and harnessing the positive effects of inflammation in specific ways and at specific times. Understanding the interaction between neuroinflammation and nerve regeneration provides several therapeutic ideas to improve the inflammatory microenvironment and promote nerve regeneration.

PXL01 alters macrophage response with no effect on axonal outgrowth or Schwann cell response after nerve repair in rats

Background: Adjunctive pharmacological treatment may improve nerve regeneration. We investigated nerve regeneration processes of PXL01 - a lactoferrin-derived peptide - after repair of the sciatic nerve in healthy Wistar rats.Materials & methods: PXL01, sodium hyaluronate (carrier) or sodium chloride was administered around the repair. After 6 days axonal outgrowth, Schwann cell response, pan- (CD68) and pro-healing (CD206) macrophages in sciatic nerve, sensory neuronal response in dorsal root ganglia (DRG) and expression of heat shock protein 27 (HSP27) in sciatic nerves and DRGs were analyzed.Results: Despite a lower number of pan-macrophages, other investigated variables in sciatic nerves or DRGs did not differ between the treatment groups.Conclusion: PLX01 applied locally inhibits inflammation through pan-macrophages in repaired sciatic nerves without any impact on nerve regeneration or pro-healing macrophages.

Multifunctional Biodegradable Conductive Hydrogel Regulating Microenvironment for Stem Cell Therapy Enhances the Nerve Tissue Repair

The nerve guidance conduits incorporated with stem cells, which can differentiate into the Schwann cells (SCs) to facilitate myelination, shows great promise for repairing the severe peripheral nerve injury. The innovation of advanced hydrogel materials encapsulating stem cells, is highly demanded for generating supportive scaffolds and adaptive microenvironment for nerve regeneration. Herein, this work demonstrates a novel strategy in regulating regenerative microenvironment for peripheral nerve repair with a biodegradable conductive hydrogel scaffold, which can offer multifunctional capabilities in immune regulation, enhancing angiogenesis, driving SCs differentiation, and promoting axon regrowth. The biodegradable conductive hydrogel is constructed by incorporation of polydopamine-modified silicon phosphorus (SiP@PDA) nanosheets into a mixture of methacryloyl gelatin and decellularized extracellular matrix (GelMA/ECM). The biomimetic electrical microenvironment performs an efficacious strategy to facilitate macrophage polarization toward a pro-healing phenotype (M2), meanwhile the conductive hydrogel supports vascularization in regenerated tissue through sustained Si element release. Furthermore, the MSCs 3D-cultured in GelMA/ECM-SiP@PDA conductive hydrogel exhibits significantly increased expression of genes associated with SC-like cell differentiation, thus facilitating the myelination and axonal regeneration. Collectively, both the in vitro and in vivo studies demonstrates that the rationally designed biodegradable multifunctional hydrogel significantly enhances nerve tissues repair.

Peripheral Mechanism of Cancer-Induced Bone Pain

Cancer-induced bone pain (CIBP) is a type of ongoing or breakthrough pain caused by a primary bone tumor or bone metastasis. CIBP constitutes a specific pain state with distinct characteristics; however, it shares similarities with inflammatory and neuropathic pain. At present, although various therapies have been developed for this condition, complete relief from CIBP in patients with cancer is yet to be achieved. Hence, it is urgent to study the mechanism underlying CIBP to develop efficient analgesic drugs. Herein, we focused on the peripheral mechanism associated with the initiation of CIBP, which involves tissue injury in the bone and changes in the tumor microenvironment (TME) and dorsal root ganglion. The nerve-cancer and cancer-immunocyte cross-talk in the TME creates circumstances that promote tumor growth and metastasis, ultimately leading to CIBP. The peripheral mechanism of CIBP and current treatments as well as potential therapeutic targets are discussed in this review.

Enhancing sciatic nerve regeneration with osteopontin-loaded acellular nerve allografts in rats: Effects on macrophage polarization

Osteopontin (OPN) is a multifunctional matrix glycoprotein with neuroprotective and immunomodulatory properties. This study explored the potential of OPN-loaded acellular nerve allografts (ANAs) to repair sciatic nerves in male Wistar rats. The research also delved into the impact of OPN on macrophage phenotypes. We reconstructed a 10 mm nerve gap with ANAs containing OPN at 2 nM and 4 nM. The sciatic functional index (SFI) and paw withdrawal reflex latency (WRL) showed the significant efficacy of ANA/OPN (2 nM) in enhancement of target organ reinnervation and subsequent sensorimotor recovery compared to other groups. Electrophysiological and histomorphometric analyses further supported the regenerative properties of ANA/OPN (2 nM). Additionally, ANA/OPN (2 nM) promoted macrophage polarization towards an M2 phenotype and reduced proinflammatory cytokines at the injury site. In conclusion, the study suggested that ANA loaded with 2 nM OPN effectively repaired transected sciatic nerves in rats, potentially through enhancing axonal sprouting and exerting anti-inflammatory effects.

Role of secretomes in cell-free therapeutic strategies in regenerative medicine

After an injury, peripheral nervous system neurons have the potential to rebuild their axons by generating a complicated activation response. Signals from the damaged axon are required for this genetic transition to occur. Schwann cells (SCs) near a damaged nerve's distal stump also play a role in the local modulation of axonal programs, not only via cell-to-cell contacts but also through secreted signals (the secretome). The secretome is made up of all the proteins that the cell produces, such as cytokines, growth factors, and extracellular vesicles. The released vesicles may carry signaling proteins as well as coding and regulatory RNAs, allowing for multilayer communication. The secretome of SCs is now well understood as being critical for both orchestrating Wallerian degeneration and maintaining axonal regeneration. As a consequence, secretome has emerged as a feasible tissue regeneration alternative to cell therapy. Separate SC secretome components have been used extensively in the lab to promote peripheral nerve regeneration after injury. However, in neurological therapies, the secretome generated by mesenchymal (MSC) or other derived stem cells has been the most often used. In fact, the advantages of cell treatment have been connected to the release of bioactive chemicals and extracellular vesicles, which make up MSCs' secretome.

Neither injury induced macrophages within the nerve, nor the environment created by Wallerian degeneration is necessary for enhanced in vivo axon regeneration after peripheral nerve injury

BACKGROUND

Since the 1990s, evidence has accumulated that macrophages promote peripheral nerve regeneration and are required for enhancing regeneration in the conditioning lesion (CL) response. After a sciatic nerve injury, macrophages accumulate in the injury site, the nerve distal to that site, and the axotomized dorsal root ganglia (DRGs). In the peripheral nervous system, as in other tissues, the macrophage response is derived from both resident macrophages and recruited monocyte-derived macrophages (MDMs). Unresolved questions are: at which sites do macrophages enhance nerve regeneration, and is a particular population needed.

METHODS

Ccr2 knock-out (KO) and Ccr2gfp/gfp knock-in/KO mice were used to prevent MDM recruitment. Using these strains in a sciatic CL paradigm, we examined the necessity of MDMs and residents for CL-enhanced regeneration in vivo and characterized injury-induced nerve inflammation. CL paradigm variants, including the addition of pharmacological macrophage depletion methods, tested the role of various macrophage populations in initiating or sustaining the CL response. In vivo regeneration, measured from bilateral proximal test lesions (TLs) after 2 d, and macrophages were quantified by immunofluorescent staining.

RESULTS

Peripheral CL-enhanced regeneration was equivalent between crush and transection CLs and was sustained for 28 days in both Ccr2 KO and WT mice despite MDM depletion. Similarly, the central CL response measured in dorsal roots was unchanged in Ccr2 KO mice. Macrophages at both the TL and CL, but not between them, stained for the pro-regenerative marker, arginase 1. TL macrophages were primarily CCR2-dependent MDMs and nearly absent in Ccr2 KO and Ccr2gfp/gfp KO mice. However, there were only slightly fewer Arg1+ macrophages in CCR2 null CLs than controls due to resident macrophage compensation. Zymosan injection into an intact WT sciatic nerve recruited Arg1+ macrophages but did not enhance regeneration. Finally, clodronate injection into Ccr2gfp KO CLs dramatically reduced CL macrophages. Combined with the Ccr2gfp KO background, depleting MDMs and TL macrophages, and a transection CL, physically removing the distal nerve environment, nearly all macrophages in the nerve were removed, yet CL-enhanced regeneration was not impaired.

CONCLUSIONS

Macrophages in the sciatic nerve are neither necessary nor sufficient to produce a CL response.