CNS myeloid cells critically regulate heat hyperalgesia

Restorative therapy using microglial depletion and repopulation for central nervous system injuries and diseases

Microglia are important resident immune cells in the central nervous system (CNS) and play an important role in its development, homeostasis, and disease treatments. Activated microglia perform diverse functions in mouse models of CNS neurodegenerative diseases or deficits. In humans, microglia have been linked to various neurodegenerative diseases. Following brain or spinal cord injury, microglia express pro- and anti-inflammatory phenotypes at different stages of recovery. With the development of pharmacological and genetic tools for microglial depletion, studies have demonstrated that microglial depletion exerts both positive and negative effects in the treatment of CNS diseases. Notably, microglial depletion provides an empty niche that stimulates production of new microglia. Microglial depletion and repopulation can not only treat diseases by eliminating dysfunctional microglia but can also provide an indication of the molecular mechanisms of diseases. Although this approach has shown impressive results, its use is still in its infancy. In this review, we summarize the current pharmacological and genetic tools for microglial depletion and highlight recent advances in microglial repopulation therapy for the treatment and functional recovery of neurological diseases and deficits. Finally, we briefly discuss the therapeutic challenges and prospective uses of microglial repopulation therapy.

Identification of sensory and motor nerve fascicles by immunofluorescence staining after peripheral nerve injury

Background

Inappropriate matching of motor and sensory fibers after nerve repair or nerve grafting can lead to failure of nerve recovery. Identification of motor and sensory fibers is important for the development of new approaches that facilitate neural regeneration and the next generation of nerve signal-controlled neuro-prosthetic limbs with sensory feedback technology. Only a few methods have been reported to differentiate sensory and motor nerve fascicles, and the reliability of these techniques is unknown. Immunofluorescence staining is one of the most commonly used methods to distinguish sensory and motor nerve fibers, however, its accuracy remains unknown.

Methods

In this study, we aim to determine the efficacy of popular immunofluorescence markers for motor and sensory nerve fibers. We harvested the facial (primarily motor fascicles) and sural (primarily sensory fascicles) nerves in rats, and examined the immunofluorescent staining expressions of motor markers (choline acetyltransferase (ChAT), tyrosine kinase (TrkA)), and sensory markers [neurofilament protein 200 kDa (NF-200), calcitonin gene-related peptide (CGRP) and Transient receptor potential vanillic acid subtype 1 (TRPV1)]. Three methods, including the average area percentage, the mean gray value, and the axon count, were used to quantify the positive expression of nerve markers in the immunofluorescence images.

Results

Our results suggest the mean gray value method is the most reliable method. The mean gray value of immunofluorescence in ChAT (63.0 ± 0.76%) and TRKA (47.6 ± 0.43%) on the motor fascicles was significantly higher than that on the sensory fascicles (ChAT: 49.2 ± 0.72%, P < 0.001; and TRKA: 29.1 ± 0.85%, P < 0.001). Additionally, the mean gray values of TRPV1 (51.5 ± 0.83%), NF-200 (61.5 ± 0.62%) and CGRP (37.7 ± 1.22%) on the motor fascicles were significantly lower than that on the sensory fascicles respectively (71.9 ± 2.32%, 69.3 ± 0.46%, and 54.3 ± 1.04%) (P < 0.001). The most accurate cutpoint occurred using CHAT/CRCP ratio, where a value of 0.855 had 100% sensitivity and 100% specificity to identify motor and sensory nerve with an area under the ROC curve of 1.000 (P < 0.001).

Conclusions

A combination of ChAT and CGRP is suggested to distinguish motor and sensory nerve fibers.

Blockade of spinal dopamine D1/D2 receptor suppresses activation of NMDA receptor through Gαq and Src kinase to attenuate chronic bone cancer pain

Introduction

Spinal N-methyl-D-aspartate receptor (NMDAR) is vital in chronic pain, while NMDAR antagonists have severe side effects. NMDAR has been reported to be controlled by G protein coupled receptors (GPCRs), which might present new therapeutic targets to attenuate chronic pain. Dopamine receptors which belong to GPCRs have been reported could modulate the NMDA-mediated currents, while their exact effects on NMDAR in chronic bone cancer pain have not been elucidated.

Objectives

This study was aim to explore the effects and mechanisms of dopamine D1 receptor (D1DR) and D2 receptor (D2DR) on NMDAR in chronic bone cancer pain.

Methods

A model for bone cancer pain was established using intra-tibia bone cavity tumor cell implantation (TCI) of Walker 256 in rats. The nociception was assessed by Von Frey assay. A range of techniques including the fluorescent imaging plate reader, western blotting, and immunofluorescence were used to detect cell signaling pathways. Primary cultures of spinal neurons were used for in vitro evaluation.

Results

Both D1DR and D2DR antagonists decreased NMDA-induced upregulation of Ca²⁺ oscillations in primary culture spinal neurons. Additionally, D1DR/D2DR antagonists inhibited spinal Calcitonin Gene-Related Peptide (CGRP) and c-Fos expression and alleviated bone cancer pain induced by TCI which could both be reversed by NMDA. And D1DR/D2DR antagonists decreased p-NR1, p-NR2B, and Gαq protein, p-Src expression. Both Gαq protein and Src inhibitors attenuated TCI-induced bone cancer pain, which also be reversed by NMDA. The Gαq protein inhibitor decreased p-Src expression. In addition, D1DR/D2DR antagonists, Src, and Gαq inhibitors inhibited spinal mitogen-activated protein kinase (MAPK) expression in TCI rats, which could be reversed by NMDA.

Conclusions

Spinal D1DR/D2DR inhibition eliminated NMDAR-mediated spinal neuron activation through Src kinase in a Gαq-protein-dependent manner to attenuate TCI-induced bone cancer pain, which might present a new therapeutic strategy for bone cancer pain.

Frontline Science: Blood-circulating leukocytes fail to infiltrate the spinal cord parenchyma after spared nerve injury

The development of neuropathic pain after peripheral nerve injury involves neuroimmune-glial interactions in the spinal cord. However, whether the development of neuropathic pain depends on the infiltration of peripheral immune cells, such as monocytes, into the spinal cord parenchyma after peripheral nerve damage remains unclear. Here, we used a combination of different techniques such as transgenic reporter mouse (Cx3cr1^GFP/+ and Ccr2^RFP/+ mice), bone marrow chimeric mice, and parabiosis to investigate this issue in spared nerve injury (SNI) model. Herein, we provided robust evidence that, although microglial cells are activated/proliferate at the dorsal horn of the spinal cord after SNI, peripheral hematopoietic cells (including monocytes) are not able to infiltrate into the spinal cord parenchyma. Furthermore, there was no evidence of CCR2 expression in intrinsic cells of the spinal cord. However, microglial cells activation/proliferation in the spinal cord and mechanical allodynia after SNI were reduced in Ccr2-deficient mice. These results suggest that blood-circulating leukocytes cells are not able to infiltrate the spinal cord parenchyma after distal peripheral nerve injury. Nevertheless, they indicate that CCR2-expressing cells might be indirectly regulating microglia activation/proliferation in the spinal cord after SNI. In conclusion, our study supports that CCR2 inhibition could be explored as an interventional approach to reduce microglia activation and consequently neuropathic pain development after peripheral nerve injury.

CXCL12/CXCR4 signaling contributes to neuropathic pain via central sensitization mechanisms in a rat spinal nerve ligation model

Background

Previous studies have demonstrated that the CXCL12/CXCR4 signaling axis is involved in the regulation of neuropathic pain (NP). Here, we performed experiments to test whether the CXCL12/CXCR4 signaling pathway contributes to the pathogenesis of neuropathic pain after spinal nerve ligation (SNL) via central sensitization mechanisms.

Methods

Neuropathic pain was induced and assessed in a SNL rat model. The expression and distribution of CXCL12 or CXCR4 were examined by immunofluorescence staining and western blot. The effects of CXCL12 rat peptide, CXCL12 neutralizing antibody, CXCR4 antagonist, and astrocyte metabolic inhibitor on pain hypersensitivity were explored by behavioral tests in naive or SNL rats. We measured the expression level of c‐Fos and CGRP to evaluate the sensitization of neurons by RT‐PCR. The activation of astrocyte and microglia was analyzed by measuring the level of GFAP and iba‐1. The mRNA levels of the pro‐inflammatory cytokines such as TNF‐α, IL‐1β, and IL‐6 and Connexin 30, Connexin 43, EAAT 1, EAAT 2 were also detected by RT‐PCR.

Results

First, we found that the expression of CXCL12 and CXCR4 was upregulated after SNL. CXCL12 was mainly expressed in the neurons while CXCR4 was expressed both in astrocytes and neurons in the spinal dorsal horn after SNL. Moreover, intrathecal administration of rat peptide, CXCL12, induced hypersensitivity in naive rats, which was partly reversed by fluorocitrate. In addition, the CXCL12 rat peptide increased mRNA levels of c‐Fos, GFAP, and iba‐1. A single intrathecal injection of CXCL12 neutralizing antibody transiently reversed neuropathic pain in the SNL rat model. Consecutive use of CXCL12 neutralizing antibody led to significant delay in the induction of neuropathic pain, and reduced the expression of GFAP and iba‐1 in the spinal dorsal horn. Finally, repeated intrathecal administration of the CXCR4 antagonist, AMD3100, significantly suppressed the initiation and duration of neuropathic pain. The mRNA levels of c‐Fos, CGRP, GFAP, iba‐1, and pro‐inflammatory cytokines, also including Connexin 30 and Connexin 43 were decreased after injection of AMD3100, while EAAT 1 and EAAT 2 mRNAs were increased.

Conclusion

We demonstrate that the CXCL12/CXCR4 signaling pathway contributes to the development and maintenance of neuropathic pain via central sensitization mechanisms. Importantly, intervening with CXCL12/CXCR4 presents an effective therapeutic approach to treat the neuropathic pain.

Microglia Express Mu Opioid Receptor: Insights From Transcriptomics and Fluorescent Reporter Mice

Background: Microglia activation contributes to chronic pain and to the adverse effects of opiate use such as analgesic tolerance and opioid-induced hyperalgesia. Both mu opioid receptor (MOR) encoded by Oprm1/OPRM1 gene and toll like receptor 4 (TLR4) have been reported to mediate these morphine effects and a current question is whether microglia express the Oprm1 transcript and MOR protein. The aim of this study was to characterize Oprm1-MOR expression in naive murine and human microglia, combining transcriptomics datasets previously published by other groups with our own imaging study using the Cx3cr1-eGFP-MOR-mCherry reporter mouse line. Methods: We analyzed microglial Oprm1/OPRM1 expression obtained from transcriptomics datasets, focusing on ex vivo studies from adult wild-type animals and adult post-mortem human cerebral cortex. Oprm1, as well as co-regulated gene sets were examined. The expression of MOR in microglia was also investigated using our novel fluorescent Cx3cr1-eGFP-MOR-mcherry reporter mouse line. We determined whether CX3cR1-eGFP positive microglial cells expressed MOR-mCherry protein by imaging various brain areas including the Frontal Cortex, Nucleus Accumbens, Ventral Tegmental Area, Central Amygdala, and Periaqueductal Gray matter, as well as spinal cord. Results: Oprm1 expression was found in all 12 microglia datasets from mouse whole brain, in 7 out of 8 from cerebral cortex, 3 out of 4 from hippocampus, 1 out of 1 from striatum, and 4 out of 5 from mouse or rat spinal cord. OPRM1 was expressed in 16 out of 17 microglia transcriptomes from human cerebral cortex. In Cx3cr1-eGFP-MOR-mCherry mice, the percentage of MOR-positive microglial cells ranged between 35.4 and 51.6% in the different brain areas, and between 36.8 and 42.4% in the spinal cord. Conclusion: The comparative analysis of the microglia transcriptomes indicates that Oprm1/OPRM1 transcripts are expressed in microglia. The investigation of Cx3cr1-eGFP-MOR-mCherry mice also shows microglial expression of MOR proteinin the brain and spine. These results corroborate functional studies showing the actions of MOR agonists on microglia and suppression of these effects by MOR-selective antagonists or MOR knockdown.